Sean Penn has a reputation for being a serious actor and a serious man. But he recently took part in a rather silly project: The Angry Birds Movie. The animated family film adaptation of the birds-vs.-pigs mobile app is a goofy affair, packed with slapstick comedy and scatological humor. And Penn was eager to sign himself up after watching an early cut of the movie.

In Angry Birds, Penn lends his voice to Terence, a character that doesn’t ever speak. Massive, stony Terence — whom lead bird Red (Jason Sudeikis) meets in anger management — isn’t the talkative type, but he utters a few grunts and growls every now and then.

Meanwhile, the other birds do speak, though that they would wasn’t an immediate given when players of the game were waiting to find out how it would be turned into a movie. (In the Finnish Angry Birds cartoon series, the birds dont have any dialogue beyond yips and yowls similar to the game.)

For a while, “it was all very top-secret whether the birds had voices or wings or legs, all those things,” The Angry Birds Movie co-director Clay Kaytis said.

Why hire Penn just to grunt and moan? Stunt casting, co-director Fergal Reilly admits. Ever since Robin Williams lent his voice to Aladdin’s Genie, casting big-name actors for animated roles has been the way to get eyeballs on your movie poster and to get audiences paying attention to and remembering your movie trailers (and hey, look, it got this movie this post on HitFix).

To not cast someone like Penn “just felt like a missed opportunity to have this great character that people really loved already in the storyboard form voiced by someone people are going to recognize,” Kaytis said.

Penn is a friend of The Angry Birds Movie executive producer David Maisel, who reached out to the actor about joining the film. After screening the film with about half of the animation complete, Penn “came out of the screening throwing out ideas for what we could do story-wise,” co-director Feral Reilly recalled.

So why did the two-time Academy Award winner decide to join Angry Birds?

“We showed him the movie, and he really, really liked it. He got the whole idea of the stunt casting,” Reilly said.

And, according to Kaytis, “He wanted to get another Oscar.”

Jokes aside, it sounds like Penn did approach the role with the same level of thought and care he would other roles. He had one recording session, and he took the full three or so hours scheduled for the session.

“He brought his professionalism and took it very, very seriously and made sure that we got all of the different nuances of Terence’s personality,” Maisel told HitFix. “There were a lot of serious discussions about Terence’s emotion and his motives and what he was trying to communicate with his grunts.”

Photo credit: Sony Pictures Entertainment Inc.

Penn had previously delved into voice acting for the English language dub of Persepolis.

HitFix reached out to Penn’s rep, and she told us he was unavailable for comment for this piece. So we don’t know whether he had played the “Angry Birds” app before choosing to join the film. His directors aren’t sure whether he had either. Though Reilly said, “He doesn’t strike you as the kind of guy who would play the game, but Sean has this raging intelligence and an inquisitiveness about everything, so I’m sure he played the game at some point.”

You can hear Penn’s growls and grunts and judge for yourself whether it’s Oscar-worthy stuff when The Angry Birds Movie opens in U.S. theaters this Friday, May 20.

An enthusiast of time travel stories, film scores, avocados and Charades, Emily Rome is an alumna of Loyola Marymount University and a native of beautiful Washington State. Emily’s writing has also appeared in the Los Angeles Times, Entertainment Weekly and The Hollywood Reporter. Follow her on Twitter @EmilyNRome.