Eddie Redmayne is already garnering Oscar buzz for his transformative performance as Stephen Hawking in The Theory of Everything.

In the romantic biopic, Redmayne embodies Hawking's physical deterioration from his diagnosis with ALS as a 21-year-old Cambridge student to his eventual loss of speech and mobility. The story is told through the prism of his relationship with first love Jane Wilde (Felicity Jones).

'But [a] greater achievement still is the emotional life of the character he projects' - Director James Marsh on The Theory of Everything star, Eddie Redmayne

Director James Marsh said in an interview Monday at the Toronto International Film Festival that the 32-year-old English actor — best known as Marius in 2012's Les Misérables — worked intensely to master the role.

"[He prepared] at great length and great detail and with great dedication and application of amazing months and months and months of physical work on his voice and how he moves, and to master a progressive disability that starts with someone who can walk to someone who's lame to someone who's lame with a stick and then with two sticks," he said.

"Each one involved details he had to totally internalize. And on any given day we would hop from one to the other, so this on a technical level is an amazing achievement."

Film's accolades are no theory 

Since the film premiered to a warm standing ovation at the fest on Sunday evening, critics have lauded both lead actors. The Hollywood Reporter said "Redmayne will be tough to beat in the best actor race," while Deadline called Jones's performance "fierce but understated."

Eddie Redmayne and James Marsh

Eddie Redmayne, left, and James Marsh attend the premiere for The Theory of Everything at the Toronto International Film Festival on Sunday, Sept. 7, 2014. (Chris Pizzello/Invision/AP)

Marsh, who won an Academy Award in 2008 for the documentary Man on Wire, also praised Redmayne for expressing Hawking's inner turmoil as he became less able to communicate verbally.

"But (a) greater achievement still is the emotional life of the character he projects out of the diminished physical circumstances the character finds himself in. That's the performance, and that works so beautifully," he said.

Jones, meanwhile, is an up-and-coming English actress known for roles in The Invisible Woman and Like Crazy. As Hawking's first wife, she conveys the conflicted feelings of a woman forced to watch the man she fell in love with undergo drastic and devastating changes.

"Felicity, I would say, is equal every step of the way," said Marsh. "And her performance has to be, as the characters in the story are so well matched in their quality of intellect and very strong wills, both of them.

"That was true of the actors too. They both need each other and the performances worked because the other actor is so good and vice versa. That's what worked on set too."

The Toronto International Film Festival runs until Sunday.

With files from CBC News