'Due Date' stars Robert Downey Jr and Zach Galifianakis and director Todd Phillips talk about making the buddy road movie.

This movie is really about fatherhood - about becoming a father and losing a father.
Todd Phillips:
Well, yeah. I think that's exactly right. You know, while it is a road movie and it is a comedy, at its core it's a movie about Zach's character, Ethan Tremblay, who's going through a trauma, having just lost his father, and Robert's character, Peter Highman, who's about to become a father for the first time. And about 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, who just needs some adjustments, I guess.

Why did that story of fatherhood resonate with you?
I think it's just an interesting take on it. For me, personally, it was an interesting movie to make. I started making movies about college kids. I sort of grow with my movies. They're always about my age range, it feels. And that's sort of the next step in life, having a kid or what have you, and fatherhood. So, it just seemed like an interesting thing to mine, both for emotion and for comedy.

Zach, is Ethan closer to your stand-up persona than the other film characters you've played?
Zach Galifianakis:
No, I don't think that Ethan Tremblay is anything like me. God, I hope not. My stand-up is more like how I am in real life. I don't really do a character thing in stand-up; it's just a bunch of sentences that are supposed to be funny. This Ethan guy is a lot more complicated, I think.

Can you talk about your take on the character and how he makes random statements that end up being funny? What are his reasons for doing that?
Well, his reasons are not intentional, I think. I mean, when you're doing stand-up, you're kind of doing, 'Hey, I thought of this. This may be funny'. But Ethan has no idea he's being funny. And I think people that are not self-aware and kind of a truck with no brakes are really kind of funny. He's saying things, but he doesn't understand why they're funny, which I think is inherently funny.

Robert, was it refreshing and a pleasure to play a character who had not been watered down but had so many real, yet repellent moments?
Robert Downey Jr:
Absolutely, and I don't know why, but it was an invitation to 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. 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. But we had such a great group of people, and the whole thing, it's funny, because yeah, you could say this is a two-dimensional commercial comedy. I feel that this is the second greatest story ever told.

The two characters in 'Due Date' strike an unlikely friendship. What's the definition of friendship for you?
I think Robert and I formed a friendship on this movie, albeit a very antagonistic but fun relationship. He's really very, very funny, and he makes fun of people a lot. And for some reason I like to be made fun of. [Laughs]

Which of you would be more likely to break up laughing in the middle of a take?
Let me put it this way. I'm 85 times more professional than Zach. [Laughs]

ZG: Yeah.

RDJ: I was hoping that we'd have some good gag reels, so maybe I'd chuckle a little bit more. He might not actually know how funny he is sometimes, too.

TP: Yeah, and Zach doesn't really break up. He just goes over his line half-way and then makes this choking sound, right?

RDJ: He has a ghastly tic. It's my favourite thing about him, to tell you the truth, particularly when we're doing press and it takes him 45 years to answer one question, when he's trying to think about what the answer is, and then he stutters and then he judges himself and then starts over. [Laughs]

ZG: I know my face is turning red. I don't want you to interpret it as being embarrassed. It's rage. [Laughs] The colour of my face is rage.

Zach, can you talk about the opportunities that have been opened up for you with the success of 'The Hangover'?
We were shooting 'Due Date' in Albuquerque last year for Halloween and I went to a Halloween party. I didn't really know anybody, and I went with a couple people from work. And I was just dressed like this. And there was a guy there dressed as the character from 'The Hangover', and I thought it would be interesting to walk up to him and say, 'Hey, you're dressed as me. I'm the real person'. And he goes, 'Yeah, right'. And he just walked away. [Laughs] So that's a little bit freaky.

And, as far as opening opportunities, well, Todd has told me of late that I've never thanked him for anything and I'm here just to say that probably he's not gonna do it today. [Laughs] Todd helped me. He took a chance, I think, and plucked me out of the stand-up scene. Nobody knows a movie's gonna be so big and we just got lucky. I got lucky and I'm thrilled that it happened.

'Due Date' is in cinemas now.