Been to hear any good films lately?

The faces are famous, but RYAN GILBEY finds that stardom has a sound, too.

The voices used to belong to unknowns, but these days no animated feature film is complete without the vocal contribution of someone from the upper branches of the celebrity tree.

It's a trade-off. The film-makers get the prestige of a big star, while the performer benefits from the ego factor: "No, of course I don't mind playing second fiddle to the animation by not showing my face. On the other hand, this is proof that I'm famous enough for the audience to know that I'm there without seeing my face."

It wasn't always the case. If it had been, then Pinto Colvig (Sleepy and Grumpy in Disney's 1937 Snow White and the Seven Dwarfs) would be a household name.

And a signed picture of Adriana Caselotti (Snow White to you) would fetch thousands.

If a well-known voice was hired, it tended to be a specialist talent, such as jazz musician Louis Prima in The Jungle Book (1967) or singer Pearl Bailey in The Fox and the Hound (1980) - well, how many other people could you plausibly cast as a character named Big Mama?

But those were the days when voice actors were required to be heard and not seen. If you noticed them, they weren't doing their job properly.

It seems impossible, now that a film such as Shrek can be sold on the strength of its star names, that all this potential was once untapped.

Part of the change has come from the new-found crossover appeal of animation: no 6-year-old will give a hoot whether Shrek is played by Mike Myers, but the difference is crucial to those cocky 15-year-olds who may bother seeing Shrek only because of Myers.

Until the rise of series such as The Simpsons and South Park, there was nothing hip about the idea of using celebrity voices. Now there are few things more credible than stepping up to the mike.

Mel Gibson demonstrated that the process could be liberating by giving his most energetic performance in years in Chicken Run.

He may be more sensitive than most about his voice, having had his Australian accent dubbed over for the US release of Mad Max (1979). What better way to exact a belated revenge than by making a mint from that formerly maligned voice alone?

The key to Gibson's performance was exactly that - it was a performance, not just a voiceover, and not a loan on his celebrity, as his role in Pocahontas (1995) had been.

Giving your voice isn't enough: you have to give everything. In Aladdin (1992), Robin Williams didn't simply provide the voice of the Genie - his physicality was there in the animation, too, as though he swam around in the inkpots and scaled each hair of the brush.

Perhaps it was the liberation of not being seen, or the challenge of creating a character through voice alone, but in Aladdin Williams rediscovered an energy from his stand-up days that few of his films had captured.

As the Genie, we saw him afresh, just as we had found in Kathleen Turner's Jessica Rabbit from Who Framed Roger Rabbit? (1988) a way to enjoy again the eroticism of an actress who had been unable to sustain her allure.

In the beginning was not, as is commonly thought, the word, but the image, and when we have grown tired of that image, animation offers an opportunity to see actors in a whole new light, an entirely different form.

Aladdin showed film-makers what could be achieved by casting stars rather than anonymous jobbing actors.

Toy Story (1996) took the notion further by using fine performers all the way down the cast list.

Below Tom Hanks you could find Laurie Metcalf as Andy's mum (a joke: she was mum to another Andy in Roseanne) and Wallace Shawn as a plastic Tyrannosaurus rex that was essentially Shawn's fussbudget character from My Dinner with Andre (1980).

Part of the attraction may be that actors can toy with their personae under cover of animation.

So Christina Ricci could further explore her sweetly malignant image by playing a psychotic doll in Small Soldiers (1998), Woody Allen could send up his neuroses in Antz (1998) and Nathan Lane could flex his warmly poisonous wit as the jealous cat in Stuart Little (1999), opposite Michael J. Fox's mouse.

The most inspired vocal casting can create sparks of recognition that bring new layers of meaning to a movie.

Disney's cleverest manipulation to date can be found in Mulan (1998), where the casting itself comments on the film's story about a girl masquerading as a male soldier.

Two of the butchest roles are taken by gay playwright and actor Harvey Fierstein (Torch Song Trilogy) and actor B.D. Wong, best known as the camp wedding designer in Father of the Bride.

What might have been throwaway in-jokes in a lesser film became subtle endorsements of the film's message: if these actors could pass for macho men, then surely anyone could change, switch, cross over.

Put like that, animation seems the ideal medium for saying the unsayable, though it's more commonly used to cram in celebrity cameos, as in Dr Dolittle 2 (Lisa Kudrow, Isaac Hayes) and Cats and Dogs (Charlton Heston, Alec Baldwin, Susan Sarandon).

There is one downside: performers have become so public about their involvement in animated features that the audience is robbed of playing guess-that-voice - a game that can be one of cinema's sweetest unsung pleasures.

One of the delights in Shrek comes not from Eddie Murphy or Cameron Diaz, but from wondering, "Who's that actor playing Robin Hood?" and then reading the end credits to find out that, oh, of course, it's ... well, go hear for yourself.

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* Shrek and Dr Dolittle 2 are showing now. Cats and Dogs is released on September 20.