Robert Downey Jr. argued with director Todd Phillips about how to shoot every scene on 'Due Date'.

The 'Iron Man' actor's co-star Zach Galifianakis reveals the pair would regularly gather in his trailer on the set of the comedy movie to fight out how a scene would be shot for "an hour and a half" before they went to work.

He said: "Robert has his opinions. And I love to watch him give his opinions because he's incredibly eloquent and he makes me laugh.

"So I would sit back while these two smart men went at it for maybe an hour and a half about how we were going to shoot a scene."

However, Galifianakis - best known for his role in surprise 2009 hit 'The Hangover'- confesses he never got involved in the discussions and instead hosted the debates.

He told the Scotsman newspaper: "Well, I didn't really want to be around, but maybe that was why they used my trailer. If it had been Robert or Todd's, I probably wouldn't have come."

'Due Date' - which tells the tale of a father-to-be (Downey Jr.) who is forced to hitch a ride with an aspiring actor (Galifianakis) in order to see the birth of his child - is released worldwide throughout Early November.