That sound you heard at 9 p.m. on May 18 was collective gasping from millions of “CSI” fans as they watched an intimate scene from the show’s season finale, in which Gil Grissom (William Petersen) chatted casually with co-worker Sara Sidle (Jorja Fox) - in a bedroom.

The two investigators are in a relationship, executive producer Carol Mendelsohn confirms. And it’s “not so new.”

She says the show’s many law-enforcement contacts say that those who work in the crime field are often attracted to each other, due to shared experiences and interests. She adds that Grissom’s relationship with Sidle is part of the character’s ongoing evolution.

“In real life, relationships take a long time to develop. Real life isn’t a soap opera. And real lives change and flow. And I think Grissom has changed over the six seasons” of “CSI,” says Mendelsohn (who, like star Petersen, is a Chicagoan).

“I think Grissom always believed that to operate at the highest level professionally, he needed to stand a distance away from people,” she adds. “He needed to be objective;, he couldn’t be subjective. But while you’re resisting all that, life happens.”

For those who might have thought the actors had to be talked into the plot development - far from it, according to Mendelsohn. The actors jumped at the chance to portray the relationship, she says, and Petersen was so into it that he even picked the shirt he wore in the scene.

Mendelsohn says the growing number of revelations about the “CSI” characters’ personal lives is a result of the actors’ desires for those kinds of stories, and also springs from a wish on the part of the writers to show what makes the investigators tick. There’s no pressure, she says, either from within the show or from the network, to make the show more like “Grey’s Anatomy,” the hit medical soap that “CSI” will directly compete with in the fall.

“We are not a soap opera. We are not a serial,” she says. “We will always be a show about science, mystery, clues, and twists and turns. And this is a twist and turn. But it’s not going to be the Grissom-Sara show from now on.”

As for the fact that serialized, soapier shows are now the hot thing, Mendelsohn says, “We have never, ever responded to external events, and I hope we don’t, because I do believe that’s when great shows stumble - when they allow external pressures to direct the story lines.”

As for the competition with “Grey’s” - a show she says she enjoys - Mendelsohn says, bring it on.

“I don’t know what’s going to happen but I do think we can both coexist on that night. And we’re going to do our best to deliver the best season ever.”

The text of my interview with Mendelsohn is below.

**There was a huge response to the Gil-Sara scene, both positive and negative. But the first question I have to ask is, is the relationship new? **

“I think the viewers are always able to draw their own conclusions about the relationships. It was a very private, intimate moment and I think it suggested a level of comfortability that comes from a relationship that is not so new.

“You talk to people in law enforcement all the time, and [the law-enforcement veterans who work on the shows or consult for ‘CSI’ shows] say law enforcement people tend to gravitate toward law enforcement people. What has always been true about ‘CSI’ and what as writers we make every effort to maintain is - there’s a reality to it. In real life, relationships take a long time to develop. Real life isn’t a soap opera. And real lives change and flow.

“And I think Grissom has changed over the six seasons. I believe that he always, from the beginning [when Grissom called in Sara in ‘Cool Change’], that it suggested that Grissom and Sara had a relationship [before she came to Las Vegas]. Whether it was platonic, whether they had once had an encounter in San Francisco - it’s a matter of debate among the writers as I’m sure it is among the fans.

**Is it accurate to say that Sara has been interested in Gil for some time, but that he sort of held her off, flirting back but still holding her at bay to some extent? **

“I would say more precisely, what is accurate is that, yes, Sara has always been interested in Grissom. A confusion existed as to whether [it was as a] father figure, friend or more than that.

“But if you look at Gil Grissom, who is the center of our show and always has been, if you look at Gil Grissom’s character from the pilot to Season 6, and you look at it as a crime scene, and look at the clues and look at the episodes, I think you discern an arc. Grissom in the pilot was much more open, more flirtatious. But over the course of the series we have seen Grissom be part of the world, withdraw from the world (which always means withdrawing from the people around him), [and] I think when he lost his hearing he retreated. When he got his hearing back I think he came back and embraced the world and got slapped in the face by all those ugly bad things out there again.

“An episode that the fans always point to is ‘Butterflied.’ There we heard Grissom express for the first time his feelings for Sara Sidle and his reticence to ever go forward and pursue a relationship. And I think that it is from ‘Butterflied’ on, that if you look Grissom, look at ‘Bloodlines,’ which ended season 4 and going into season 5, [that is] where Grissom embraced his team…

“I think Grissom always believed that to operate at the highest level professionally, he needed to stand a distance away from people. He needed to be objective, he couldn’t be subjective. But while you’re resisting all that, life happens. The connection that Warrick and Nick expressed for Grissom in ‘Mea Culpa,’ when Grissom lost his team, is something that Grissom felt too. Unexpressed, albeit. You can say I don’t want to be a family with these people, I want to keep my distance, but the truth is, over time, he got very close to them and he cared for them.

“And I think that ‘Mea Culpa’ and ‘Grave Danger’ - the love of Grissom’s team basically brought Nick to life. And I think that Grissom evolved even more Season 6. The straw hat’s as perfect example. Grissom realizes life is short, Grissom realizes he is closer to the end of his career,  at least at CSI - [though] he’s still a young man - than he is to the beginning of his career. I think that he is suddenly of this world, cares about his people, and I think that slowly he allowed himself to open up to Sara.

“Obviously we are not a soap opera. We are not a serial. We will always be a show about science, mystery, clues and twists and turns. And this is a twist and turn. But it’s not the Grissom-Sara show from now on.”

**There are a fair number of fans who commented on my site that they want the show to be about the casefiles, not ****necessarily ****about relationships.  **

“I think we support those fans. The beauty of ‘CSI’ is, yes, it is about the mystery and the science and the cases, but one of the other factors that has made us a fan favorite, what has made our fans so loyal, is that we have the best cast on television. And the characters that have been created by those actors and with the writers and with Anthony Zuiker, they’re wonderful characters and people love them. And we sometimes stop for a moment, and it is just a brief moment, to allow those characters to live and breathe and express who they are, and I think that has been true from the beginning.

“A little always goes a long way on ‘CSI,’ but a little always seems to be magnified on ‘CSI.’ And the show has always told us what works and what doesn’t work. We have, over the seasons, written many more character scenes that the fans never saw. We shot them, and we looked at them in the cutting room, and looked at them in the context of an entire episode, and they didn’t work.

“I believe very strongly that Grissom would like people to believe that his brain is bigger than his heart. Because that makes it easier for him. But I think, when you are passionate about anything, and he’s passionate about his career, that makes you capable of passion in other parts of your life. And I think that he does care and he does feel and that is what makes him a great CSI.

“And I really think that the scene is more about who the man is. This is a relationship that the audience is privy to but no one else at CSI is privy too. We decided [to do this] as writers, and after lengthy conversations with Billy and Jorja, because we would never go into this kind of scene without [consulting them]. As with everything on ‘CSI,’ it’s a collaboration. We believe it is grounded in reality, and I don’t mean to speak for Jorja and Billy, but to see on television a relationship between mature, adult professionals is interesting.

“It doesn’t mean that we’re going to revisit it every week, but it means that we let the audience in, to get a glimpse of a very private Grissom. A Grissom with his clothes on, albeit he was lying in a bed, he was fully dressed [laughs]. But they never really touched.”

Billy Petersen is signed for one more season of “CSI,” right? Can you foresee a “CSI” without him?

“You’d have to ask Billy. I know he is signed for another season, beyond that I don’t know. We all love Billy, and we all love Grissom. And there are some things in life you just don’t want to think about. It’s too hard to contemplate. We’ve always said to Billy, when you want to leave, let us know, let the writers know, and he’s never said that. It’s amazing that going into season 7 we have our entire cast intact. That’s a testament to them. I assume if we have a very, very long run that we will lose some of our actors. But that’s a long way of saying, I can’t think that far [laughs].

“Sometimes a great show has a heart that beats independently of all of us, the writers, everyone. … I think the show’s greatness exists apart from any one of us, or perhaps all of us. Can I say the show will go on? I think it will. Will the joy be gone in Mudville if we can’t write for Billy and Grissom? Oh boy, yes.

“But at the moment the show is about Grissom and his team, and I hope it will always be about that.”

You talked about consulting with Billy on that scene. Has there been a desire on his part to show more of who Grissom is, show more of his personal life?

“After the hiatus, you should get in touch with Billy, he will definitely have insights on that. When Naren Shankar, the co-showrunner, and I went to Billy and said we would like to do this, he got so excited. I think it felt natural for him. It felt natural for Grissom to be going to this place. And Billy enjoyed the idea of portraying a mature, adult relationship. It’s not people falling into bed. It’s real life. He was excited, excited down to, he picked the shirt he wore. Billy and Jorja were really excited about it.

“As writers and as fans - I almost can’t look at the screen. I think Grissom is my Grissom, and I think we all feel that way. When Grissom almost kissed Lady Heather, I couldn’t watch. The writers are very much in the same boat as the fans.”

**Can you clear one thing up - did Grissom actually sleep with Lady Heather? **

“I think it’s for the fans to intuit what their relationship actually is. I can say, it’s an ongoing debate among the writers. There are two camps - among everybody, cast, crew and everybody at ‘CSI.’ There are those who believe that yes, they did sleep together, and there are people like me that believe that they had an intimate encounter of some kind, but it was not necessarily sexual. Billy may be of two minds about it himself. Listen, there certainly is a connection to Lady Heather, she is an intellect and a scholar in her own way. But we saw the dark side of that this season. But you would think he would have a very different connection with Sara. He can talk about bugs with Sara.”

How romantic is that?

“I think for a forensic entomologist, it is!”

**So next year you guys are going up against “Grey’s Anatomy”? What are your thoughts on that? **

“We have been through this once before, when Les Moonves [head of CBS] moved us from Friday to Thursday. Anthony Zuiker and Jerry Bruckheimer and Billy were like, ‘Bring it on.’ I was a little concerned, but I learned from that. I learned that two great shows can coexist on a night. There is enough audience. I believe that it’s a lot of competition for us, it makes next season very exciting for us. It’s a great show. I don’t know what’s going to happen but I do think we can both coexist on that night. And we’re going to do our best to deliver the best season ever.”

**We’ve been talking about the fact that there has been more about character’s personal lives in the last season or two of “CSI” - is that in any way a response to the success of shows like “Grey’s”? **

“No. Our response on ‘CSI’ is never to external events. I think a show starts to struggle when it’s reactive to external events, as opposed to growing and evolving from what is happening organically in the show.

“The writers and actors are very collaborative on our show, and it is a testament to their acting that they are able to pick up a fiber, swab a blood sample and make it look so interesting and engage our viewers. It was really to allow our actors, who are such great actors, to do a little more. A chance to… I don’t want to say exercise their chops… It was a request. It was something that was obviously very important to our actors. As the show grew and we went on season by season… I think that we’ve focused a little more, in small doses, on [the question], who are these people that we see every Thursday night? We’ve always tried to do it in an organic fashion and never have it substitute [for the mysteries].”

“[A good example is] Sarah Goldfinger’s ‘Rashomama’ -  you got to have fun with our characters, but it was such an engaging mystery. And that is what we try to do.

“We have never ever responded to external events and I hope we don’t, because I do believe that’s when I believe great shows stumble - when they allow external pressures to direct the storylines. Of course, if I read about something in the newspaper, that doesn’t stop us from being inspired by things that happen in the world. But I’m just saying, increased competition, shows are serialized, soap operas are back in fashion - I have never believed that that works [following the trend] and that’s something we’ve never done on ‘CSI.’

“As a fan, I want to know a little more about them. I think we’ve done very effective stories revealing Catherine’s backstory and tying her to Vegas. And that has been a theme throughout the season, Old Vegas and New Vegas.

“But Grissom has always been a very private person and this is an instance where we allow the fans to look through a window into a private moment.”

**Would it be fair to characterize what you’re saying as - it’s the natural evolution of the show and it’s the desire of the actors to have that different flavor of those personal scenes to play? **

“It is. I think it would surprise the fans to know the many conversations and the days we spend talking about the characters on ‘CSI.’ We especially do it at the beginning of every season and at the end of every season. You can’t write a show unless you talk about who these people are and what makes them tick. We don’t always show what makes them tick, but when a situation presents itself, when a story lends itself to that, it’s exciting for both the writers and the actors to have a little blip of the private person.

“There is a delicate balance on ‘CSI’ always, between the personal and the case. We try to keep that balance. I think the fans would say maybe we go a little to one side, maybe to the other side. But I do think an episode like ‘Rashomama,’ the fans liked it. We got to see what our characters thought about marriage. There is a place for character development on ‘CSI,’ we always have to handle it carefully.

“Sometimes maybe the writers tend to go overboard, but [between the writers and the actors, etc] it’s like the checks and balances of government.

There was a huge response on my Web site and on many others regarding the finale. Have you been experiencing that, have you heard or read the feedback from fans?

“Like anybody else, I go on certain Web sites, and there were lots of e-mails, and from talking to [‘CSI’ writer] David Rambo - the fans got his e-mail, and so he has received quite a lot of pro and con mail. And from CBS, I’ve heard there’s been quite an outpouring. Going into the seventh season, I love the fact that everybody cares about our show.”