Due Date is a slow build of a comedy, and it’s effective at that. By the time it gets really crazy, you’re so waiting for it that it’s a cathartic release through the rest of the movie. It also means that the first half could be a bit of a chore if you’re not in it for the long haul.

Peter Highman (Robert Downey, Jr.) isn’t an entirely likeable guy. Keeping in touch with his wife by Bluetooth is fine, because a man’s got to work, but he does seem to court disaster with his attitude towards other people. Ethan Tremblay (Zach Galifianakis) is an oblivious screw-up, as frustrating as he is lovable.

So the first half of their adventure is full of normal mishaps: an airport misunderstanding, running out of money, an automobile accident. Peter faces an ethical dilemma when he considers leaving Ethan, and Ethan has an emotional stake in the road trip too, because he’s got his father’s ashes. Due Date starts there and then builds to crazy.

Peter actually softens after he’s survived the worst of things, but to appreciate that you have to see him flip out for a while. Ethan starts to say more random non sequitur lines about Shakespeare, and there is more slapstick on the bumpy road. The stunts are huge, really eye popping setups that make you worry for the characters. You’re probably never worried for their life, but you’d worry how on earth they’ll get out of the latest situation when it seems so debilitating.

Sounds like an instant riot, right? To get there is gradual. The first automobile accident really plays the reality of an injury. It’s kind of intense, certainly not comedic slapstick. Peter and Ethan really bite at each other, and the emotional attacks really cut deep. You expect a comedy to throw in the heart, not go all Precious: Based on the novel Push by Sapphire on you. You need that to appreciate when they get goofy though.

The forces that push them together make sense and build up to outrageous extremes. It takes the exact right exchange of words to get them kicked off a plane, but it seems natural, not contrived. Ethan is relentlessly unhelpful, but again, Peter doesn’t exactly smooth situations over provoking the people whose help he needs.

Some of the early jokes aren’t as funny as they think they are, like Peter riffing on baby names or his chauffeur butting into the conversation. That, I’m just going to have to call it like I see it. It does get crazy funny though. People will say it’s not as funny as The Hangover but The Hangover only works because of the tonal shifts too. Due Date has different sorts of tonal shifts but it commits and gets to a good place.