In one of the most unexpected movie comebacks ever, Brendan Fraser transformed into a morbidly obese man for "The Whale" that premiered at the Venice Film Festival Sunday and made him an instant frontrunner for an Oscar.

One of the biggest names in Hollywood in the 1990s, Fraser starred in a string of family friendly blockbusters as a muscle-bound hunk in films like "George of the Jungle" and "The Mummy", before his cinema career fell off in the late 2000s.

But his return as Charlie, a 600-pound (250-kilo) English professor trying to reconnect with his daughter, has drawn instant acclaim.

"I needed to learn to absolutely move in a new way," Fraser told reporters in Venice.

"I developed muscles I didn't know I had. I even felt a sense of vertigo at the end of the day when all the appliances were removed, as you do stepping off the boat here in Venice."

The movie is the latest from US director Darren Aronofsky, who won the top prize, the Golden Lion, in Venice in 2008 for "The Wrestler", and is known for dark indie films such as "Black Swan" and "Requiem for a Dream".

"The Whale", however, has a profoundly optimistic attitude.

Praise for Fraser

As the story unfolds, the reasons for Charlie's obesity emerge -- rooted in trauma linked to his sexuality and religion -- but he never allows them to darken his view of the world.

"Charlie is by far the most heroic man I've ever played," Fraser said. "His superpower is to see the good in others and bring that out in them."

Aronofsky said this was "the most important message to put out in the world".

"Everyone's leaning into the cynicism and darkness and giving up hope, and it's exactly what we don't need right now. We have to lean into (the idea that) underneath it all, we really do care about each other," the 53-year-old director said.

While the film drew some criticism for being a "soupy melodrama", in the words of Screen Daily, there was near-unanimous praise for Fraser's performance.

"The Brendanaissance is on! Fraser gives a towering performance, in every sense of the word," wrote IndieWire.

Deadline said "a never-better" Fraser is now "at the front of the Best Actor race".

'Lighthouse in a dark sea'

There have been some grumblings in the US press about whether an obese person should have been cast in the role.

On that point, Aronofsky said: "I considered everyone, every kind of movie star on the planet, but none of it ever clicked.

"Then, a couple years ago, I caught a trailer for a Brazilian movie, kinda low budget. Brendan had a part in it and a lightbulb went off."

"The Whale" was originally a play by Samuel D Hunter, who helped adapt it for the screen.

"I wrote this character from a very personal place. I have a history of self-medicating with food. I was a gay kid in a fundamentalist religious high school," he said in Venice.

"I was afraid to write it. The only way I could do it was write it from a place of love and empathy... I wanted him to be a lighthouse in a dark sea."