CANNES, France (AFP) - Twilight actress Kristen Stewart blasted the downside of superstardom but thanked Woody Allen for uncovering her hidden depths as his new film opened the Cannes film festival on Wednesday.

Stewart, who has premium billing at this year's event with two films in the main showcase, delighted critics with her performance in Allen's Cafe Society, a romantic romp set in 1930s Hollywood.

As she completes the transition from teenage vampire star to indie movie darling, Stewart said that massive fame at a tender age had been a heavy price to pay for her current artistic freedom.

"It's like the most gnarly popularity contest in the world," said Stewart, whose relationship with Twilight co-star Robert Pattinson attracted intense media attention.

"You take high school and make it like in the real world - it amplifies everything, it's pretty intense."

Stewart, 26, and her co-stars Jesse Eisenberg and Blake Lively were peppered with questions about a line in the film that says: "Hollywood is boring, nasty and dog-eat-dog".

"There's definitely an undeniably-opportunistic, hungry, insane fervour that occurs," Stewart said.

"I think human beings are always clawing at each other to get on top. I think that's true in most industries but Hollywood can have a surface nature that makes it more obvious."

Stewart said she had auditioned for the part in Allen's film - a rare move for an actress with her strong box office draw - and had to abandon her usual approach to a role.

"Luckily, once we got going, that tonal quality that's so familiar and immediately recognisable (in Allen's films), it just happens intrinsically. Who knows, maybe we didn't do it but I think we kinda nailed it," she said with a sly smile.

Stewart said she worked with Allen to develop the character, a small-town secretary who wins two men's hearts in Hollywood.

"It's never completely changing myself but it's like finding things that are a little more buried. He probably saw something in me that I didn't and that's the best feeling in the world."

Lively, displaying a slight baby bump in a crimson red jumpsuit, said Tinseltown had evolved since the glamourous era depicted in Allen's film.

"Back in the 30s, the studios were probably a bit more dominating than they are now, they owned actors and filmmakers," the 28-year-old said.

"Now I think it's the media that is more dog-eat-dog and invasive and the access people have to knowledge - if they don't have access they'll just make it up. Now it's probably more challenging."

Stewart, who will also appear in the competition film Personal Shopper during the 12-day festival, chimed in: "That's a good point."

Eisenberg, 32, whose private life has attracted less intense speculation in the press than that of his female co-stars, said earning a name in the movie business had been "very useful" to him as an artist.

"Fame is the probably most valuable currency, for better or for worse," said the "Social Network" star.

Allen took the long view on his more than six-decade career, warning stars against taking themselves too seriously.

"There are great, great upsides to it (fame) and great downsides and my own opinion after years in the spotlight is that the perks far outweigh the downsides," he said.

"Celebrities often kvetch about the lack of privacy and the amount of paparazzi, things like that - these are not life-threatening problems and they get enormous advantages as they go through life."

Asked about his remarkable longevity, Allen said he even surprised himself.

"I'm 80, I can't believe it. I'm so youthful, agile, nimble, spry, mentally alert that it's astonishing," he said.

"I'm sure one day I'll wake up in the morning and I'll have a stroke or something. Until that happens, I'm just going to continue to make films as long as people are foolish enough to put up the money to support me."