ZACH GALIFIANAKIS as Ethan Tremblay and ROBERT DOWNEY JR. as Peter Highman in Warner Bros. Pictures' and Legendary Pictures' comedy DUE DATE.

BEVERLY HILLS -- Just as anyone who has seen their characters in Due Date might expect, Robert Downey Jr. and Zach Galifianakis really don't get along that well.

"I think Robert and I formed a friendship on this movie -- albeit very antagonistic," Galifianakis says of his unlikely partner in the anti-buddy roadtrip comedy, opening nationally in theatres on Friday.

"He makes fun of people a lot, and for some reason I like to be made fun of."

Downey Jr., sitting next to his co-star at a news conference, admits that when he first teamed up with Galifianakis on the set of Due Date, he felt it was his "duty to teach this guy how to f---ing act," but soon learned Galifianakis already knew how -- even if he does have a "ghastly tick.

"It's my favourite thing about him, to tell you the truth, particularly when you're doing press and it takes him 45 years to answer one question. He's trying to think about what the answer is, and then he stutters and then he goes, 'Mmm hmm,' and then he judges himself and then he starts all over again," Downey Jr. says before launching into a full-fledged parody of Galifianakis screwing up his lines on set -- much to his frenemy's dismay.

"I know my face is turning red," Galifianakis responds. "I don't want you to interpret it as being embarrassed ... it's rage. The colour of my face is rage."

Galifianakis may blush, but he doesn't crack up. Downey Jr. says he's "85 times more professional than Zach" but was more likely to burst into laughter while shooting Due Date.

The film directed and produced by The Hangover's Todd Phillips and Dan Goldberg sees the Iron Man star's wallet-less architect, Peter Highman, forced to ride from Atlanta to Los Angeles with The Hangover star's aspiring actor, Ethan Tremblay, in order to be with his expectant wife (Michelle Monaghan) for the birth of their first child.

As with any road-trip comedy, much ridiculous hilarity ensues as the mismatched pair -- Peter a practical thinker with anger-control issues, and Ethan a free-spirited stoner who shares inappropriate habits with his French bulldog Sonny -- make their way through the States.

But beneath the driving disputes, death-defying accidents and numerous pit-stop cameos (Juliette Lewis, Jamie Foxx and Danny McBride are among those Downey Jr. and Galifianakis' characters cross paths with) lies a theme of fatherhood -- to which Peter, a soon-to-be-dad, and Ethan, whose pops recently died, can both relate.

"While it is a road movie and it is a comedy, at its core it is a movie about (Ethan), who's going through a trauma, having just lost his father, and (Peter) who's about to become a father for the first time," Phillips says. "And why they needed to meet at this moment and why Robert needed to travel with this kind of man-child who was going through this traumatic experience, but really is a purely loving creature, much like a child would be."

Galifianakis -- best known for playing common-sense-challenged brother-in-law Alan in The Hangover -- even takes a serious turn, as Ethan opens up to Peter about his late dad in a scummy bathroom. He says such moments are "fun to do ... if you can make people a bit emotional watching a scene, and then make them laugh."

Downey Jr.'s character also softens in that particularly touching scene -- a rare moment given that Peter is the type of guy who yells first and asks questions later.

"I don't know why but this (character) was an invitation for me to get in touch with everything that annoys me about everyone, and all the fear I have about everything that everybody can relate to -- so in a way I felt like I was a conduit to this," says Downey Jr., whose recent career resurgence includes bigtime roles in Iron Man and its sequel, and Sherlock Holmes and its upcoming sequel.

"It wasn't very pleasant because I'm not a method guy; I can't be bothered to have a method. I just want to be part of a good movie and I can't stand being surrounded by morons."

In this case, Downey Jr. isn't referring to Galifianakis. Honest!

"We had such a great group of people. It's funny because you can say this is a two-dimensional commercial comedy, (but) I feel that this is the second greatest story ever told."

And the first one?

"Oh, come on! The Bible."

Downey adds child punching, dog spitting to his resume

Robert Downey Jr. has gotten more flak for spitting in a dog's face than he has for hitting a kid in the gut in the new comedy Due Date, hitting theatres Friday.

"I love that we've actually gotten more pushback from spitting in a dog's face than from punching a human child in the stomach," says Downey Jr., whose temperamental character Peter loses it on the bratty son of Juliette Lewis' character Heidi in one scene of the film, and on Zach Galifianakis' character Ethan's French bulldog in another.

Both dirty deeds were for the sake of comedy, Downey Jr. adds.

"Todd told me to do both," he says, shifting the blame to The Hangover director Todd Phillips. "He said, 'You should spit in that dog's face ... I want you to spit in that dog's face.' I said, 'That's so definitive.' And he said, 'But I think people will like you enough -- or I don't know if they will. Just spit in that dog's face.'

"He loves dogs and I actually don't like dogs, so I felt kind of horrible and splendorous doing it."

lindsey.ward@sunmedia.ca

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