Ben Stiller ‘I had this small part in Steven Spielberg’s WWII movie Empire of the Sun and some friends of mine were in Hamburger Hill. I was sharing stories about fake boot camps and actors having these life changing experiences. It just seemed there was some humour in actors taking themselves too seriously and not really knowing what they’re doing most of the time. To me that was funny.’

Jack Black ‘My character’s a portly gentleman named Jeff Portnoy who usually does lowbrow comedy films, to do with gaseous expulsions from the rectal region. He’s trying to make a career change by doing this Vietnam War drama, Oscar-worthy type film. There’s a little homage to an Eddie Murphy film, and a lot of different characters that I’m drawing from; a little Chris Farley, a little Tom Sizemore, a little me.’

Stiller ‘Jack was always the first choice to play the archetypal funny, gross out guy, because he would be able to make fun of stuff that he’s done before and yet make the role his own.’

Black ‘I took to an M60 machine gun like a baby takes to milk. I don’t know why. I’ve never shot a gun before. But they handed me the M60, known as the pig ‘cause it’s so big, and I was running through the fields shooting at targets. I was shooting blanks, but still very macho looking.’

Stiller For Kirk Lazarus, it was important to have somebody who actually was respected as one of the great actors of their generation and who’s really funny, too.

Robert Downey Jr ‘Playing a white actor named Kirk Lazarus who plays a black man named Sergeant Lincoln Osirus, even though you have some aesthetic distance by the way it’s set up, it’s still potentially hazardous. But the whole premise of the movie ‐ Ben as this disaffected action star and Jack as the frat-boy fart gag comedy guy ‐ there would be something missing if somebody wasn’t going so method that it was really inappropriate and borderline psychotic.’

Black ‘Taken out of context it might be inflammatory, but if you see the movie you’ll just marvel at Robert’s abilities. He’s got the hardest role, because he plays the greatest actor on the planet, a kind of Daniel Day-Lewis type, and an African-American character who is the ultimate test of Kirk Lazarus’ talents. It’s a very funny tour-de-force.’

Downey ‘I was just in the zone. There were some days where I literally felt I was Sergeant Lincoln Osiris, which is stupid because the whole thing I was making fun of was that Kirk Lazarus thinks he is this character. But there were days where to all intents and purposes externally I was black. I’m in my trailer at lunchtime and people would come by and it was easier just to stay in the character than to break it.’

Stiller ‘We had to be really clear what our point of view with this character was and where the humour was coming from. It’s an actor who’s making this wrong-headed decision to go too far to play an African‐American guy, which is just a ridiculous idea for a white actor. So it was all about lampooning actors who take themselves too seriously.’

Black ‘It’s a fun look at the industry itself. Kind of taking the piss out of Hollywood.’

Downey ‘Given an opportunity, and in the right company, I do it all the time, anyway.’

Tropic Thunder is released on Fri 19 Sep.