In the mother-son road trip comedy "The Guilt Trip," opening Dec. 19, Seth Rogen plays an organic chemist who worked for the Environmental Protection Agency and is now trying to market a sustainable nontoxic cleaner he invented made from coconut, soy and palm oils. He invites his overbearing mother, played by Barbra Streisand, to accompany him on his cross-country pitch trip, secretly hoping to reunite her with a lost love. Screenwriter Dan Fogelman, whose story was inspired by a real-life road trip he took with his mother, borrowed that plot point from a friend. "He was starting an organic cleaning supply company made of all natural food products and he would tell me about the process of trying to get it sold across the country, and how you do it. We actually had a scientist create the formula that might create a real product."

There's a different kind of chemistry at work between the leads, who hit it off instantly and improvised a lot of the dialogue. "The 10 biggest laughs in the film, more of the comedy stuff in the car, is all stuff Seth and Barbra came up with," notes Fogelman. Director Anne Fletcher couldn't see any other actors in the roles, and she wasn't going to make the movie unless both signed on. "There was no backup," she says. Rogen did so first, and made two movies in the year or so it took for Streisand to commit.

While she's had supporting roles in the "Fockers" movies, Streisand hadn't done a lead role in 16 years. "I don't make that many movies, and I don't make that many appearances. Less is more. And maybe that keeps a little mystery or something. I like to stay home a lot. I like to do other things, too, like decorate, build," she says. "I made it very difficult for them to hire me because I kept wanting an out in some way," but they agreed to accommodate the late riser with the call times and locations convenient to her Malibu home. (In fact, the whole film was shot in Southern California, supplemented by second unit-shot backgrounds that were blended in.) But what clinched it for Streisand was her son Jason's encouragement. "I really trust his integrity and his opinion. He has great taste."

For her, the most challenging scene involved consuming a giant steak, as she doesn't particularly like it. Fortunately, substitutes were found so she wouldn't get too sick of it. Otherwise, no complaints, and she relates to the mother-son dynamic. "It's a transformative kind of movie. They start at one point, both of them kind of tragically alone, not finding a mate. And then at the end there's many more possibilities. He took me out of my shell. It was a very loving gesture. I always say it's a different kind of love story."

"The Guilt Trip" rings true for Rogen as well. "My mom drives me crazy sometimes. I have a good relationship and see my parents a lot, but it's a lot like in the movie ... that dynamic where your mother's trying and the more she tries, the more it bugs you. And the more it bugs you, the more she tries."

Fogelman believes that viewers of all ethnicities will relate to Streisand's character Joyce. "Everybody should be able to look at Barbra in the film and be reminded of their own mother." Streisand would like it to have even more of an effect on audiences. "I want them to be moved. I want them to identify. I want them to see themselves in the movie," she says. "I want them to get closer to their children."

As for her future plans, she'd like to do a stage revival of "Gypsy" and plans to direct a film about Margaret Bourke-White and Erskine Caldwell with Cate Blanchett and Colin Firth. Rogen will play a family man who lives next door to a rowdy fraternity house in "Townies," opposite Zac Efron, and makes his directorial debut with the apocalyptic comedy "The End of the World."

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