Contrasting personalities, Peter Highman(Robert Downey, Jr.) and Ethan Chase (Zach Galifianikis) are heading from Atlanta to Los Angeles. En route, plain and simply put, shit happens. Every Hiccup in their journey is a hiccup for you, because you want Due Date to cut to the chase.
Peter Highman says to Ethan Chase "Now hear this, beware, I'm warning you, don't go to Hollywood. The streets are not paved with gold, they're paved with the carcasses of fucking imbeciles like you who think they're gonna go there and make it. Pray you avoid it. Did you get that?" This dialogue is pitched perfectly. You know why? Because Robert Downey Jr. wasn't aware that he was being filmed. The person he was talking to was director Todd Philips.
Downey Jr. stands apart from the crew like a butterfly among roaches. His performance is effortless and genuine because just like his character, he feels trapped in the situation. Peter Highman finds Ethan Chase a pain in the ass. You empathize because you feel the same way about the film.