For the Despicable Me prequel Minions, filmmaker Pierre Coffin had no trouble finding A-list actors to voice characters because so many are fans of the earlier films. And most are looking for the freedom offered by starring in an animated movie.

MinionsVoicing characters is much simpler than performing onscreen

Jon Hamm voices the groovy inventor Herb, married to super-villain Scarlet Overkill (who's voiced by Sandra Bullock). He loved creating an all-new character. "You're not constrained by what you look like or what haircut you're stuck with from whatever thing you just finished filming," he says. "Part of it for me was getting the artist's rendering of the character. I was like, 'Oh wow, that guy looks so cool. I wonder what he sounds like.' And then messing around with voices in your head like a crazy person, or like a child basically. You know, things I do normally in my day-to-day life, like I talk to my dog, I talk to myself. It's really fun and it's completely creative. And when you're working with people who are also incredibly collaborative too, they're just like, 'Go!' So you get to go."

Watch the trailer for 'Minions' here

Allison Janney, who plays aspiring villainess Madge, approached with role with an open mind. "I just came in ready to work," she says, "just doing all different kinds of crazy voices and trying to find the right voice for Madge, which we found. I love that she's my first evil character, even though you wouldn't think she's that evil. Madge is a good mom. She just happens to rob banks."

More: Sandra Bullock liked playing the villain in 'Minions'

The one drawback with animated films is that you don't meet your costars until the film premieres. "One of the things I love about acting is other actors, looking into their eyes, listening and responding to them," Janney says. "This kind of work is a little strange. When I first saw Jon at the premiere, he was like, 'You're in this?' I saw Michael Keaton getting out of a limo, and he goes, 'There's my wife!' I didn't know what he was talking about, and he told me that he was playing my husband. But the thing that's great about it is I can go to work in my sweatpants. I'm a sweatpants and T-shirt kind of girl. I love to be in bare feet when I work. I have to be willing to make a complete fool out of myself."

More: Read our review of 'Minions'