IT seems apt that cinema’s latest Odd Couple met in less than entirely friendly circumstances.

But when Zach Galifianakis, breakout star of the 2009 comedy hit The Hangover, noticed Robert Downey Jr having a coffee with his wife, he couldn’t resist introducing himself – to Downey’s evident confusion.

“I approached Robert on the street and told him we were going to be doing a movie together,” Galifianakis explains.

“I tend not to recognise people,” Downey adds, “and I didn’t know exactly who Zach was yet.

“I guess I looked like I was going to punch him.”

From that unpropitious moment to the unbridled, raucous comedy of Due Date, the latest from The Hangover director Todd Phillips.

Downey plays Peter, an uptight architect racing cross country to attend the birth of his first child.

Galifianakis is his unwelcome travelling companion Ethan, a good hearted innocent with a knack for landing them in trouble.

The comedy is as risqué and out there as you would expect from the participants, but it also raises the question of how far is too far in a high grossing gross-out comedy?

“I think with Todd’s movies the whole point is to go too far,” Galifianakis explains. “In comedy you have to do that sometimes, because so many beats have been done in movies before.

“I think it’s important not to draw a line,” writer-director Todd Phillips adds, “because that line will be drawn for you by audiences once you start putting the movie on its feet. But when we’re actually shooting it’s not like ‘ooh, what’s too far, and what’s not?’. We don’t rein it in, we tend to just go for it and then shape it in the editing room.

%movie(15694)

“There was this one thing that we all agreed was hugely outrageous,” says Downey, “this moment in act three when I finally asked him ‘how did your Dad pass away, I never knew?’.

“And Zach turns to me and says ‘oh, sharing needles’. We were thinking ‘that’s so wrong’, but Todd says ‘well let’s see what the audience thinks’. And sure enough, they were like ‘that’s sick, I’m going to walk out’.”

So that particular line was duly cut. Due Date is the sort of film you laugh along with despite your reservations about moments where good taste is not so much ignored but bad taste heartily embraced.

It’s a film to clear the cobwebs and, ultimately, has a good heart of its own.

“One of the things I love about Due Date,” Downey smiles, “is that it starts when I get kicked off the plane and then the guy says ‘the person you’re travelling with’ and I say ‘I’ve never met that [idiot] in my life’ and he says ‘he had nothing but nice things to say about you’.

“Todd said ‘just to make it definitive that you don’t want to hang out with this clown any more just spit in his dog’s face’.

“It was right after lunchtime and I said ‘no, isn’t that going too far?’ and he said ‘spit in the dog’s face, I love dogs, he doesn’t mind’.

“So I spat in the dog’s face and Sonny was just like ‘Oo, what happened? Did he have to get that out of his throat or something? I hope he feels better’.

“I was just thinking: ‘My God, what is wrong with us?’”