EXCLUSIVE: Bryan Cranston Says Working With James Franco on 'Why Him?' Was the 'Most Fun' He's Had on a Movie

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Only ET can take you to the set of the upcoming comedy, Why Him?, where stars Bryan Cranston and James Franco gushed about the experience.

"I can tell you unequivocally it was the most fun I've ever had shooting a movie," Cranston told ET.

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In the movie, Cranston plays Ned, an overprotective-but-loving dad who develops a rivalry with his daughter's socially awkward billionaire boyfriend, Laird (Franco), which reaches epic proportions when he learns that Laird plans to pop the question.

Thanks to a hilarious script and surprising improvisations, the actors admitted that they both broke character several times while filming, but they explained that it's not always a bad thing.

"If you are actually cracking up on set and that's your job, it's amazing," Franco said. "But I never want to lose a great line or a great reading, so I really try not to laugh."

Cranston said that Franco might have been better about keeping a straight face on camera, which could be due to the fact that the film's storyline hits closer to home for Cranston. The Golden Globe winner and his wife, Robin Dearden, have a 23-year-old daughter, Taylor.

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"[I'm] very protective," Cranston said. "It's the one thing when you become a father. It's like, 'That's it. There's finally something in this world that is far more important than myself or my wife, and that's everything."

Why Him? opens Christmas Day. Franco says he'll be spending the holiday with his two brothers, Dave and Tom, and their mother, Betsy.

"None of us have children, so it feels like this is a holiday that needs some children soon," Franco said. "My youngest brother is actually getting married in February. Maybe he'll be the first."

READ: Alison Brie and Dave Franco Reportedly Engaged

Dave Franco, 31, is engaged to actress Alison Brie. The couple announced their upcoming nuptials in August 2015.