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How do you turn a 12-page script into a 78-minute comedy?

The prolific indie director Joe Swanberg ("Drinking Buddies") did it in "Happy Christmas" by picking good actors who have astonishing improv skills.

It also helps to cast your own two-year-old. Swanberg cast his son Jude to play the son of Jeff, a filmmaker played by Swanberg.

The movie — screening in the U.S. Dramatic competition of the 2014 Sundance Film Festival — centers on a Christmas season in which Jeff and his novelist wife Kelly (Melanie Lynskey) play host to Jeff's sister Jenny (Anna Kendrick), who's emotionally messed up after a recent break-up.

In a Q-and-A after a Tuesday morning screening of the film, Swanberg explained that Jude was cast at the perfect age — old enough to walk and be verbal, but young enough to not say "no."

"He's really a different kid, who doesn't want to do anything I want to do," Swanberg said.

Swanberg shot on Kodak film, something he wanted to do before film technology is replaced with digital cameras.

Shooting on film is difficult when your actors improvise their dialogue, but Swanberg said it made his cast focus. "They knew we couldn't do a lot of takes," he said.

Several of the characters smoke pot, but the cast didn't partake of the real stuff while shooting (something that did happen with Swanberg's last movie, the alcohol-heavy "Drinking Buddies").

"They told me, 'You will get one take out of us, and we will be out for the rest of the day,' " Swanberg said.

— Sean P. Means