TIFF: Eddie Redmayne and capturing the smiling eyes of Stephen Hawking

How does one find the right actor to play Stephen Hawking?

Typically, directors and producers do their homework by reviewing an actor's previous work, looking for clues in the on-screen moments to inspire confidence that a performer has the goods to breathe life into their character. Actor X can do this because he already did that. But unless Daniel Day-Lewis is in the mix, there really is no film resume that suggests an actor can convincingly portray Hawking, the brilliant British scientist whose body is ravaged by amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease), the motor-neurone disease that robbed him of his ability to speak on his own.

"[In casting Hawking], you have to take a leap into the dark," says director James Marsh, who successfully gambled on Eddie Redmayne, immediately propelling the 32-year-old British actor and their film, The Theory of Everything, into Oscar contention after its world premiere at this week's Toronto International Film Festival. Redmayne, best known in the U.S. for playing Marius in Les Misérables, and falling hard for Michelle Williams in My Week With Marilyn, wowed critics and audiences alike, playing the charming and mischievous academic whose physical condition slowly deteriorates from the moment he's diagnosed with ALS in the 1960s. In a still-evolving Best Actor race that also includes Foxcatcher's Steve Carell and The Imitation Game's Benedict Cumberbatch, Redmayne might be the one to beat.

"We always have that feeling of euphoria [when we get a role], followed by a few days later of, 'Oh, I've actually got to do it,'" says Redmayne. "In this case, the euphoria lasted under a millisecond [after I hung up the phone], followed by deep, terrifying, overwhelming fear. But [co-star and friend] Charlie Cox (Boardwalk Empire) said to me, when he heard I got the part, 'The great thing about this job is that it's one that you can do nothing but give 3,000 percent. That's not an option.' And that was totally true."

Redmayne and Felicity Jones, who plays Hawking's first wife, Jane, spent nearly six months researching their roles, meeting with the Hawking family and other ALS patients. Redmayne went to great lengths to prepare his body for the rigid demands of the role, working with an osteopath, who helped him study old photos of Hawking to determine the rate of physical decline, and a dance choreographer, who then incorporated those conclusions into his body behavior: the tightly twisted joints, the crumpled posture, the facial contortions and vocal difficulties. "It's the wasting of muscles but some of it's rigid, so it's not about just playing it like this," Redmayne says, making his face slack.

And then it happens, almost like magic. As Redmayne continues speaking, he effortlessly flexes the subtle facial muscles that transform him into Hawking. We might all have these same facial muscles, but you've never witnessed them in action. It is mesmerizing.

"When some facial muscles stop working, Stephen finds more. So when you meet him now, even though he can move very little, it's one of the most expressive faces. All this around his eyes. I spent a lot of time in front of a mirror, learning to try and use muscles that I haven't used before."

Thus, Redmayne came to the set uber-prepared. But it almost appeared as if Marsh wanted to make things more challenging. For one, they shot out of sequence. That's not unusual for a large production, but it certainly added to Redmayne's assignment. "The first week, we had to shoot all our Cambridge exteriors, and interiors too sometimes," Marsh says. "That meant in the very first week of the shoot, on any given day, I was expecting and asking Eddie to be able to provide and offer progressive disability: he had to walk, he had to be on sticks, and he had to be in a wheelchair."

The actor developed a grading system of sorts that measured Hawking's physical condition, allowing him to quantify the degree of deterioration—say, a 4.0 performance versus a 2.5 performance—and then be able to amp up or tone down the physical handicaps from scene to scene. Before long, the fluctuating but relentless physical demands began to take a toll, and Redmayne's own face began to be impacted by the stress. "Stephen speaks a lot from [the right] side of his face, and throughout the film, I basically got stronger muscles [there], and Jan, our amazing makeup designer took notice," says Redmayne. "It was all quite uncomfortable—but at the end of the day, I could get up, and so many of the [ALS patients] who let us into their lives can't."

End of the day, yes. Middle of the day, not so much. Marsh and Redmayne laugh now, but the director preferred that his star stay in the wheelchair for extensive periods of time, a request that the actor understood. "It was about sculpting—oh, this sounds f—ing pretentious—a kind of sculpting from within rather than just replicating a thing [on the surface]," says Redmayne.

The strategy worked, but not without pain. "I had a window into that punishment that Eddie did to himself every day," Marsh says. "I'd watch rushes and you'd see him sort of just break out at the end of a take. [Makes a strained exhale gasp] You'd see him sort of leap back into a bit of Eddie, but there was also some Stephen left lingering. It was something I saw every day. And I began to feel guilty about it—'Oh my God, what am I doing to him? He's going to hate me soon.'—but not that guilty.

Hawking and his family have seen the film, and he liked it enough to give the filmmakers permission to use his actual computerized voice. Redmayne became friendly with Hawking's children, especially his youngest, Tim. Winning their approval weighed on Redmayne, who did not want to disappoint or embarrass Hawking. So as much as he's grateful for the overwhelmingly favorable reception in Toronto and the media's increasing chatter about an Oscar, Redmayne aimed for a positive review from a family of select critics. "Tim wrote a lovely text message the other day, in which he described how [he and his sister, Lucy] were watching the movie, and at the end, when Stephen gets up, they both said the fact that they could see, for a second, what their father may have looked liked able-bodied was incredibly moving for them. And in turn, very moving for me."