At this point, Zach Galifianakis knows a thing or two about how to make an unstable character funny. “People who are like a truck with no brakes are inherently funny,” says the actor when discussing his role in his new film, “Due Date.”

The film, which reunites Galifianakis with his “Hangover” director, Todd Phillips, offered him the chance to go one-on-one with Robert Downey Jr., who stars as a high-strung father-to-be forced to hitch a ride across the country with a disastrous aspiring actor (Galifianakis) in order to make it to his child’s birth on time.

“It is a road movie, [but] at its core it’s about Zach’s character having just lost his father and Robert, who’s about to become a father for the first time, and why they needed to meet at this moment,” Phillips offers, in a moment of surprising sincerity.

The theme of fatherhood was appealing to Phillips, who is better known for films about men clinging to their faded youth. “I started making movies about college kids and I tend to grow with my movies; they’re always about my age range,” he says. “That’s the next step in life — fatherhood. It seemed like an interesting thing to me, both for emotion and comedy.”

For his part, Downey Jr. was also in on the parenthood theme. “Every time I feel I reached a point of change is when the director and I become a third thing, and that’s the character,” he says of working with Phillips. “I always feel I’m playing an aspect of the director. It’s a way of making him a proud parent.”