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Before they signed on to do the buddy comedy Due Date, Robert Downey Jr., 45, and Zach Galifianakis, 41, had never met. But after making their rollicking road movie, the pair now finish each other's sentences and are relieved not to be spending any more time trapped in a car together. The stars sat down with PEOPLE's Jennifer Garcia to talk friendship, fatherhood and what they just can't stand about each other. But first Downey, carrying a whoopee cushion-like toy, let it rip.

DOWNEY: I don't usually have toys with me. But I love the idea of a guy who always has a whoopee cushion with him wherever he goes.

GALIFIANAKIS: You should carry a whoopee cushion so that if paparazzi follow you, you can pretend to start answering a question and say, "well, that's interesting . . ." and then just let the thing blow.

You guys spent a lot of time filming together. What's the most annoying thing about your costar?

DOWNEY: He has a tic-

GALIFIANAKIS: I thought I had that tic removed! Is this going to be embarrassing? Is it my breathing?

DOWNEY: He doesn't like foul language anywhere near the workspace. Whereas my F-bomb count was embarrassing.

I find that hard to believe.

DOWNEY: Absolutely true! But he has this other tic: When he stumbles on a line, he gets stuck, and it sounds like somebody who's off their medication and is really upset.

GALIFIANAKIS: He's right. And well-spoken.

Zach, how did you come up with your girlish strut in this movie?

GALIFIANAKIS: Well, I think the tight jeans I wore had a little something to do with it. The crampage-it was created out of discomfort.

DOWNEY: What about the Capezio dance shoes you wore, did they help?

GALIFIANAKIS: You know what's strange? Everybody is talking about the Capezios, and nobody is talking about the Lilith Fair T-shirt I also wore.

DOWNEY: By the way, to him, the T-shirt was the reveal of the movie.

How did you bond off-camera?

DOWNEY: I wanted to rehearse, but far be it for me to cut into Zach's Chablis time-

GALIFIANAKIS: I've never had Chablis in my life, but it sounds good.

Robert, you're a dad [to son Indio, 17]. Do you think about parenthood, Zach?

GALIFIANAKIS: I think about it a lot. The one thing in the last 10 years of my life that I have wanted to be is a father. But more than them being teenagers, I have a real problem if, like, a child fell off the couch and hit their head. That's the big worry for me. Everything else doesn't bother me.

DOWNEY: So just as long as the things that are inevitable don't occur, you'll be all right?

GALIFIANAKIS: That's right.

DOWNEY: Okay, but you can't baby-proof the west side of Los Angeles! You should practice with a doll first.

GALIFIANAKIS: A baby that is made out of nerf. A nerf baby!

DOWNEY: That is the best advice ever.

Downey, who has a son from a previous marriage, says he and his producer wife want to have kids: "We decided to do it." Galifianakis has been dating Lundberg (right), a former ballerina and cofounder of the charity Growing Voices, for over a year.