As hard-core fans of CBS’ “CSI: Crime Scene Investigation” know, former CSI boss Gil Grissom (William Petersen) had deaf parents and therefore is adept at American Sign Language. They also know he married fellow CSI Sara Sidle (Jorja Fox) and is in the jungle doing research, while Sara is back in Las Vegas working crime scenes.

But what they probably don’t know is that Grissom had another woman in his past, something that Sara discovers in a rather uncomfortable way.

In the episode “The Two Mrs. Grissoms,” scheduled to air on Thursday, Feb. 3, after a murder at Gilbert College, a school for the deaf, Sara is caught between her mother-in-law, Betty Grissom (Phyllis Frelich), a professor at the school, and Julia Holden (Marlee Matlin), another deaf professor who doesn’t know Mrs. Grissom just from work.

“The relationship between me, Julia and Sara is interesting,” says Matlin, speaking sign language through an interpreter between scenes, “because there are some surprises that arise during the episode. I know her husband; we had a past; we were intimate.”

Matlin, an Oscar winner, has an extensive background in both TV and film as well as strong local ties. She grew up in Morton Grove, graduated from Hersey High School in Arlington Heights and attended Harper College in Palatine.

Her “CSI” role puts her in conflict with Sara.

“(Sara) has a lot on her plate, not only about her husband but trying to figure out if I’m the murderer or not,” Matlin adds. “She’s straddling the fence as to whether she should trust me as a woman who says she didn’t do anything wrong, because there’s a conflict there.”

Complicating the situation is Sara’s testy relationship with her mother-in-law.

“Phyllis is an amazing actress,” says Fox, “and one of the first things she said to me was, ‘I’m sorry. I’m going to be mean to you on the show.’

“A couple of times, when my character has turned to walk away, there have been looks that could kill. I haven’t seen them, but they’ll probably make it into the final cut.”

Adding to Sara’s misery is co-worker Nick Stokes (George Eads), who has transferred his admiration for Grissom to his mentor’s mother.

“I invented this whole back story,” Eads says. “I’m overtly gracious to Mrs. Grissom when I see her, almost like she’s a surrogate mom. Because of the regard he has for Grissom, he thinks the same of the mom.

“So I’m playing a little bit where I’m Mrs. Grissom’s pet, and Sara’s kind of the problem child. Nick likes seeing Sara in an awkward and precarious position.”

While Nick may be kissing up more to Mrs. Grissom, Eads may not be working as hard on his communication skills.

When it’s mentioned that both Eads and Fox were learning ASL for the show, Matlin quips, “Not so much George.”

On the other hand, regarding Fox, Matlin says, “She’s so enthusiastic, and she’s so eager to learn more new signs. She seems to be a perfectionist, and she wants to sign how her character would sign. She’s not supposed to know sign language skillfully, so what she knows fits her character. It’s not like she sees her mother-in-law every day.”

“I think I have the proficiency of a clam right now,” says Fox of her ASL chops. “I’m hoping to work my way up to squirrel by the time we finish this episode. It’s sure something I’d like to continue with. “Sara has been making an effort to spend time with her mother-in-law, especially because Grissom’s not in Vegas. She’s been practicing and learning sign language, but we all know that Sara’s a workaholic, and maybe she hasn’t quite focused on that to the degree that Mrs. Grissom would like.” Even though Petersen is no longer on the show, the character of Grissom has been kept in everyone’s minds, especially in this episode.

“When we were talking about doing this show,” Fox says, “I did call him, because I wanted to make sure that he was comfortable with this.”

While there’s no word whether Julia might ever return on “CSI,” Matlin has had a good experience. “This is really one of the best jobs I’ve ever had,” she says. “It’s just too short. It’s a family here, and I’m entering their territory. But they are so welcoming, with open arms, open hearts, open minds.”