OK, I will admit that I used to think that Due Date was only a so-so movie. Now I have revised my opinion, and I think its pretty good. That Ethan Tremblay character was just so annoying yet funny and Peter Highman was such a jerk yet funny. Reminded me of that movie 'Plains trains and automobiles' . I now see inteviews with the actors together and I wonder if they got along very well in real life, I have read some stories about how the two first met and how they eventually started getting along and I wonder if Hollywood has sort of played things up between the two actors..?
I've read that RDJ thought a disheveled Zach Galifinakis was a homeless person the first time they met. Zach tells the story that he made a faux pas by taking a couple of bottles of wine as a gift the first time he had dinner at RDJ's house! Then, apparently the two hit it off quite well. There are interviews where RDJ says he very much liked Zach and all the cast/crew of Due Date and that filming it was a happy, positive experience.
Based on that, I'd say that, after a rocky beginning, the two got along just fine.