If it weren't for the fact that it's awfully hard to stage a car crash and a car chase in most theatres, I'd say that Due Date would make a perfectly good two handed stage show. It's absolutely anchored in the interaction and reactions of the two lead actors. Luckily, they went out and got two lead actors who could carry it off.
It's not a great movie; it's not even a great script. But it carries it off, just, because the leads are up to the challenge.
Robert Downey Junior is having, as I've said before, one of the great second acts of an American acting career. He is not, however, just playing to his familiar strengths. Since he came back out of oblivion, he's been playing various likeable characters who have problems. In Due Date, he's playing an unlikeable character. He's not a bad person; it's just that he's tense, and judgmental and far too impatient with everyone. He tries to be relaxed and calm and nice to people, but you can see how it's an effort (there's an early scene with a limo driver where you get a perfect sense of the character; Downey makes no effort at all to connect with the driver; the limited conversation is obviously something he's just enduring till he can get out of the car and never see him again). This is not quite the loveable Downey we've seen in other things.
Zach Galifanakis keeps weirding me out when I see him. It's because I first saw him as the pathologist in Tru Calling, where he was playing a pretty straight role as an educated grown up with a responsible job. So it's been odd watching him in the Hangover and Due Date as an overgrown flabby man-child with absolutely no sense of what's allowed for grown ups. Due Date's Ethan Tremblay is nothing all that new for anyone who's seen The Hangover, although it is different. This is a much sunnier character, though equally out of his depth in the real world. What's astonishing is the way Galifanakis sells it. It's far harder than it looks to play someone too stupid to realise that he's stupid (there's a standout moment when Ethan dismisses the idea that the Grand Canyon could possibly be a natural feature - it takes real genius to get an audience to buy the idea that there's anyone dumb enough to stand in front of the Grand Canyon and STILL think someone dug it out).
And that's the whole job done, boys and girls; the interactions between the blissfully deluded Galifanakis and the uptight Downey. Its drags on a bit at times, but it has moments of real power; there's a scene when Downey leans down to comfort Galifanakis after a truly disastrous incident with Galifanakis's father's cremated remains. Downey's character can't stand Galifanakis at this point, but it makes perfect sense that in a moment this bad, he would transcend his dislike. Another actor might not have made it work; Downey nails it.
And two thirds of the way through it kicks everything up a gear in comedy terms; there's a jailbreak cum car chase which is such magnificent physical comedy that I just burst out laughing at the beginning and kept laughing all the way through. It was a long time building up to the punchline, but once it took up a notch, for a while, Due Date was one of the funnier things I've seen this year.