On the Set : The Chemistry Is No Mystery : PRODUCER OF CBS FILM SAYS KEN OLIN AND PATRICIA WETTIG PLAY LOVERS WITH A REAL SPARK

The way actors Ken Olin and Patricia Wettig met and fell in love is so classic, it’s almost corny. “We were, believe it or not, doing ‘Streetcar Named Desire,’ ” says Wetting. “I know, it’s so ridiculous. It sounds like I made it up. But we were doing Stella and Stanley in ‘Streetcar’ in Portsmouth, N.H. We met at the train station in New York City, took the train up and spent a couple of months in the dead of winter in Portsmouth.”

“We just fell in love with each other,” says husband Olin later, “and we’ve lived under the same roof ever since.”

Ironically, 13 years of marriage and two kids later, they’re acting the part of lovers for the first time since Portsmouth--in a “CBS Tuesday Mystery Movie” titled “Nothing but the Truth.”

Viewers will remember Olin as Michael Steadman and Wettig as Nancy Weston on the critically acclaimed series “thirtysomething.” While both actors were married on the show, it was to different on-screen spouses. They haven’t appeared together since “thirtysomething” went off the air in 1991.

In “Nothing but the Truth,” Wettig plays professional polygrapher Jill Ross. An expert in her field of lie detection and a university professor, she is assigned to test murder suspect Dr. Peter Clayman, played by Olin. Dr. Clayman is the town’s handsome new internist. After he successfully passes Jill Ross’ examination, the two become romantically involved.

But as the murder investigation into the death of a young woman proceeds, Clayman’s innocence is again questioned. Ross at first defends her professional skill and her lover vigorously, but eventually doubts both.

It’s a steamy thriller a la “the Joe Eszterhas model,” says executive producer Stan Brooks, but with an unexpected twist. “Part of what excited all of us involved was the ability to take a genre which might be a little tired at the moment and turn it on its side and see if we couldn’t breathe a little fresh life into it.”

The film’s outcome will surprise viewers, adds Brooks. “It’s a very unusual ending for this genre.”

On the set in March, it is pouring rain, Southern California style. The UCLA campus is standing in as the New England college where Ross holds a faculty post. Wettig, radiant in a plaid skirt and peach sweater, waits patiently for rehearsal of the scene to begin, singing Lisa Loeb’s “Miss You” quietly to herself. Olin stands with her, looking every bit the murder suspect, hair cropped short, his body leaner and more defined since the last days of “thirtysomething.”

The two run lines together while the cameras set up. From a distance, nothing indicates that these two actors know each other intimately. But every now and then, there is a tell-tale sign. “I shouldn’t button this, should I?” Olin consults Wettig as he fiddles with his shirt collar. “Mmm. Let me see it buttoned,” responds Wettig. For a moment, they are every husband and wife preparing to go to work.

But what happens in front of the camera, says Brooks, is very rich. “What we’ve gotten, because they know each other so well, are those subtle moments in eye contact that I don’t know that you would get with two actors that just met each other.”

For both Wettig and Olin, “Nothing but the Truth” represents a career departure.

“I get tired of the true-life tragedy-victim characters on television, which I’m offered all the time,” laments Wettig. “You know, well, this happened to this person, it’s a true story, or this happened and she fought back, and this person, she was abused ... I liked the idea of doing a piece of fiction. And this is a murder mystery--it’s not an area I’ve ever worked in before.”

Brooks initially brought the script to Wettig after meeting her for another project. “She’d come into my office,” says Brooks, “and I sat across the table from her and was mesmerized by how beautiful she is and realized that no one had ever fully taken advantage of that on film. She hadn’t been allowed to be sensual, sexual and she really is.”

For Olin, who has successfully embarked on a directing career, “Nothing but the Truth” marks his return to acting after a three-year hiatus. “I have been very determined to pursue my career as a director,” says Olin, whose directing credits include the Fox TV movie “Doing Time on Maple Drive,” the feature “White Fang II” and the recent HBO movie “In Pursuit of Honor.”

“I felt one of the best ways to learn is through experience, so I’ve done a lot of things in a relatively short period of time and different things so I could learn. I wanted that to be my exclusive commitment. And also I felt that it was really important to be taken seriously as a director, that I don’t consider directing a vanity.”

But for both Wettig and Olin, the primary draw of “Nothing but the Truth” was the opportunity to work together. “It’s not very difficult for me to do a scene where I’m taken by him or in love with him and moved by him,” says Wettig of working opposite Olin. “Here it’s just natural.”

“I love working with her,” says Olin. “I’ve never worked with a better actress. She’s really good. She’s really good.”

“Nothing but the Truth” airs Tuesday at 9 p.m. on CBS.