Mark Ruffalo on Cinematic Spouse Trading

Mark Ruffalo doesn't advocate wife swapping, that's just what he does on the big screen, and something reality TV-happy Americans enjoy watching.

In We Don't Live Here Anymore, Ruffalo and Peter Krause of Six Feet Under play New England college professors — best friends who end up in bed with each other's wives.

Infidelity has always been a popular theme in popular entertainment. But these days, given the success of Fox's Trading Spouses and ABC's upcoming Wife Swap, it's bigger than ever. Yet Ruffalo wanted to give audiences more than a voyeuristic thrill.

"In America a little more than 50 percent of marriages end in divorce," says Ruffalo, who felt so strongly about the script, based on two short stories from In the Bedroom author Andre Dubus II, that he spearheaded the project as executive producer.

"These people are all struggling with one of the most difficult endeavors we may encounter in this life," he says. "The suffering was honest, the problems and people were complex and the dialogue beautifully written."

In the film, Ruffalo's character and his headstrong wife, played by Laura Dern, are caught up in the never-ending cycle of bills and chores. After an evening of free-flowing wine and conversation, he and his friend's wife, played by Naomi Watts, sneak off to a convenience store parking lot. And after an unexpected kiss, two marriages are in jeopardy.

Immediately suspicious, Dern confronts her husband, and tells him that his buddy had actually made a pass at her when he was out of town. But that only makes Ruffalo feel free to continue his affair and tell his wife as little as he pleases.

"I wonder how we'll get caught," Watts muses after a naked romp in the woods, as the extramarital games kick into high gear.

"It tells it like it can be at its worst," Ruffalo says. "Yet it leaves room for the simple grace of one's ability to work back to the original love that made it come together in the first place."

The 36-year-old actor is having quite a year, and quickly becoming a familiar face, even to occasional moviegoers. In Collateral he's the scruffy detective chasing Tom Cruise. In 13 Going on 30, he's Jennifer Garner's hapless boyfriend. And in Eternal Sunshine of the Spotless Mind, he brainwashes Jim Carrey.

It's quite a recovery, after facing a brain tumor three years ago, just after a breakthrough performance staring opposite Laura Linney in You Can Count On Me.

The condition, never considered life-threatening, forced Ruffalo to opt out of an opportunity to play Mel Gibson's brother in M. Night Shyamalan's Signs and retreat to upstate New York, where he needed 10 months to recover from treatment that caused temporary paralysis to his face and medication that ballooned his weight by 40 pounds.

But the experience allowed the busy actor to spend more time with his son Keen, now 3, and his wife, Sunrise Coigney, whom he married four years ago. And he's returned with a new passion for acting.

Of his character, Ruffalo says, "He is coming to a point in his life when he believes the best of him has been left in the past. He feels time ticking away — he sees it in the river moving, the continuing schedule of the town around him, the train.

"He has lost touch with his wife, and he learns to lie about his unhappiness, as well as what he believes are her shortcomings. He tries what so many men at midlife do. He tries to recreate himself by destroying his past."

In his next role, Ruffalo may have to deal with another character's checkered past. He's set to appear in something of an update of The Graduate, starring Jennifer Aniston as a woman who discovers her grandmother was the inspiration for the character of Mrs. Robinson — and that she just might be the lovechild born of that classic romp.

Here's to you, Shirley MacLaine, for taking on the character immortalized by Anne Bancroft and forever celebrated in song by Simon & Garfunkel.

The still untitled film, to be directed by Rob Reiner, is due to hit theaters in 2005, when infidelity, most assuredly, will certainly still be a hot topic.