When Ben Stiller and Chris Rock were first asked to become a talking lion and a cartoon zebra, neither had toddlers to impress or PTA colleagues to please.

Stiller was busy strutting down fashion runways as the "really, really, ridiculously good-looking" Derek Zoolander. Rock was doing some strutting of his own, across the stages of his sold-out comedy shows.

So why'd they sign up for Madagascar, an animated movie about escaped Central Park Zoo animals on the lam in Madagascar?

"It just seemed fun," Stiller recalls, now seven years later on the opening of the sequel yesterday, Madagascar: Escape 2 Africa.

And it was fun, as it turned out. Fun for him, fun for Rock, fun for millions of moviegoers and more than half-a-billion dollars' worth of fun for DreamWorks.

If you want to know where Hollywood is hanging out these days, check that DreamWorks studio. Or its competition down at Pixar. Or at the back entrance to each, banging on the door to get in.

Doing voice work for animated films has never been a more popular career move for sought-after stars, one that's enjoyable, shows some winking playfulness, offers very little risk and huge potential for upside. Think Mike Myers, Eddie Murphy, Robin Williams, Jerry Seinfeld, Renee Zellweger and Tom Hanks.

Chris Rock has a six-word philosophy for picking film projects: "Just go with the best movie."

Thus, he signed on to Madagascar and its sequel simply because "they're good movies!" Makes no difference if it's live action, animated or Claymation; it's not method that matters, he says, but the material. "I'm a comedian," he says. "Anything that's funny I want to be involved in. ... This is definitely as funny as any live-action film I've been involved in."

That might come as a surprise to fans of Rock's trademark envelope-pushing adult humour. But he and Stiller say it's the fact that these kid-centric movies are so rife with high-level comedy that makes them enticing -- to them as actors and to the grown-ups forking over cash at the box office.

"It's fun sometimes for adults to be watching a quote-unquote kids movie and see a joke that they think, 'Oh, wow, I can't believe they actually put that in there,' " Stiller says. "... And the kids, it'll go right over their heads."

This kiddie flick is densely populated with actors known for their decidedly adult fare. Listen for the voices of Alec Baldwin, Andy Richter, Cedric the Entertainer, Sacha Baron Cohen of Borat fame, The View's Sherri Shepherd and the late Bernie Mac, who plays the noble dad to Stiller's long-lost lion, Alex. (This time around, the four "zoosters" find themselves encountering animals of their own kind after a penguin-piloted plane crashes in the savannahs of Africa.)

It's not just a voice that actors are asked to give to their animated counterparts. The work of creating a life is required as well.

When actors such as Rock and Stiller show up to work on an animated film, the movie is little more than a comic-book-like set of images and a very basic script. That means going in blind to an unusual extent, but it also allows a degree of creative control that might not be possible on a live-action film.

In the seclusion of a recording booth, actors do take after take after take of each scene, the dialogue and delivery going in any direction they choose. The animation, eventually, is made to match, and because cameras are trained on the performers the whole time, their expressions and movements may also be reflected in the finished product.

"It makes you have an ability to take more chances, because you know the animators can work with it ... or come up with ideas based on what you're doing," Stiller says. Actors doing this kind of work need to have "a willingness to take chances and not worry about looking a little ridiculous sometimes in the recording studio."

"It's weird, a little bit, too -- 'cause you're alone the whole time," Stiller adds. And he means alone. Though Rock's and Stiller's characters spend much of the 89 minutes of screen time together, the two spent only one day recording in the same room.

But make no mistake: This IS acting, Rock says. Especially after the animators come back later in the process to show the performers how a scene is working on the screen. "You're acting comes on immediately. You're like, 'Ooooh, I can do that one better. I can put a little more emotion in that one,' " he says. "I mean, did you see Kung Fu Panda? Dustin Hoffman is acting. That's the best Hoffman performance I've seen in a while."

Animated movies can take years to complete (the first Madagascar took five; the second one wrapped in 3 1/2), but it's not heavy lifting for the actors, at least compared with live-action films. Stiller and Rock may have gone into the studio for 15 or 20 days for each movie, for three or four hours at a stretch, but that was it. No makeup, no costumes.

The commitment they make is to come back to the project as many times as necessary to record new dialogue or do another take on a scene. Rock ended up doing zebra jokes over the phone for the first one. And when Stiller was in Hawaii filming Tropic Thunder, he dropped into a studio to do some Alex the lion audio. "It's sort of like a process of remaking the movie over and over again until you get the best version you can," Stiller explains.

And he was right, about the whole thing being fun. So much fun that he's teaming with DreamWorks to produce an animated film about a villain in search of a superhero to antagonize. (It was written with Robert Downey Jr. in mind, but if that doesn't work, Stiller may do the voice himself.) And Rock, who did the voice of a mosquito in Bee Movie for DreamWorks, says they're talking about the possibility of a Mosquito Movie.

Rock has become the father of two since the Madagascar adventure began, as has Stiller. So the benefits they never envisioned at the outset are proving to be some of the sweetest. "My youngest daughter likes to brag," Rock admits. "Any party we're at, by the time the party's over, every kid knows I'm the zebra from Madagascar.'"