Take an old Volkswagen bus prone to breaking down, fill it with a family of assorted characters — including a foul-mouthed, drug-snorting grandpa — move it along the interstate and you’ve got “Little Miss Sunshine.”

But the Sundance smash-turned-B.O. hit really isn’t a dysfunctional-family-on-a-road-trip movie, says Alan Arkin, who plays the Hoover clan’s cantankerous elder.

“It’s about these specific individuals, and they’re very carefully and brilliantly drawn,” the 72-year-old thesp says. “It’s about unusual, wonderful people in pain who end up recognizing that they’re tied together in deeper ways than they are separated.”

The family’s goal is to get young Olive (Abigail Breslin) from their home in Albuquerque, N.M., to Southern California after she gets an unexpected shot at a national beauty pageant.

Like the plot, the 30-day shoot also turned into a singular mission.

“When we were in the van, it was 95 degrees in the desert, there was no air conditioning, and we were in there, sometimes, for three or four hours at a time,” Arkin says. “But everybody’s head was so into the movie. We were getting along so well and so intent on trying to maintain what was actually a pretty difficult tone to maintain.”

En route, each of the characters faces a personal crisis, and Gramps chimes in about all the goings-on — whether anybody asks his opinion or not.

“I love completely disreputable characters who spout philosophy,” Arkin says. “He’s completely out there, wide open and nuts. I just loved the character.”

So did critics, including Ty Burr of the Boston Globe, who wrote: “If the character were played by anyone other than Alan Arkin, he’d be unbearable, but he is played by Arkin, so he’s bluntly hilarious.”

Despite Arkin’s fondness for the character, he came to the Fox Searchlight pic with some apprehension as it was directed by first-timers Jonathan Dayton and Valerie Faris, who cut their teeth on musicvideos.

“One doesn’t get particularly excited about hearing you’re working with somebody on their first feature,” he says. “And then I wasn’t particularly excited about hearing there was going to be two directors. And then the third thing that wasn’t exciting was to find out that they were married.

“I went into it with a little bit of trepidation, but that lasted about two days, and it became clear that all of my anxieties were unfounded. They were imaginative and full of ideas and open to other people’s ideas. They were kind of a dream team.”

Favorite film of the past five years: “Three movies of Iranian director Majid Majidi blew us away: ‘Baran,’ ‘The Children of Heaven’ and ‘The Color of Paradise.’ ”

Actor who impressed you greatly after working together: “I can’t limit it to one. When I did ‘Glengarry Glen Ross,’ I spent a lot of time watching Al Pacino work. I liked watching his rehearsals. Harrison Ford is a man after my own heart because he never leaves the craft service table. I loved working with Nick Nolte, dedicated an actor as you’re ever going to run across.”

Next project: “Raising Flagg” is due in February, and Arkin’s voice work for next year’s “Bee Movie” was completed months ago.