Keira Knightley and James McAvoy in Atonement Courtesy of Focus Features

Wright conducted a three-week rehearsal period with the cast, allowing all the actors to meet each other even if they weren't in scenes together. To get into character, McAvoy read the script as many times as possible before filming started but found the rehearsal period particularly invaluable.

"The acting process in this film wasn't about just getting a bunch of British actors who could do stiff upper-lip accents and put them in costumes and let them go, because they all know what they are doing" McAvoy says. "It really was about getting a bunch of people together, putting them around a table for three weeks and galvanizing them… So it was collective experience, actually, which you don't always get in films. Quite often it is a bit disparate: I've got my trailer, and you've got your trailer, and we'll see each other when it happens. It was never like that in this."

Research into the Second World War was encouraged. To capture the stylized look of the 1930s period, McAvoy says Wright "very boldly want[ed] us to go some way towards not only behaving like people in 1935, but acting like the actors of 1935, which isn't a hundred percent representative of the way society was. It is slightly heightened which I think is appropriate for what this film was about."

Aside from investigating acting methods from that era, voice warm-ups were a "big thing."

"We were never allowed to be off-voice," McAvoy recalls. "There was no whispering-acting going on unless one was dying. There was no dropping of energy [with one's] lines. It was all quite theatrical techniques. It was about getting the energy to the other actors, who would then give it back to you so the energy builds in the scene. Especially when you have a very restricted and internalized form of acting, and when it might in some ways seem that there is not a lot going on, you need that energy to drive the scene. And of course with all that internalization of such huge emotions, when it does come out, it explodes."

In the telling of the film, Cecelia and Robbie reunite. They meet for the first time in six years in a teashop scene, and when Briony appears at her sister's doorstep, both Robbie and Cecelia confront her with the past.

Keira Knightley and James McAvoy in Atonement Courtesy of Focus Features

"[These] were the two scenes that I responded to the most," McAvoy says. "And they were the two scenes that were the hardest because they are the two scenes where the character and the actor must become the most vulnerable… Technically, other scenes were more difficult, but emotionally those were the hardest."

While those scenes may have provided McAvoy with the greatest challenge as an actor, it is the steamy sex scene with costar Knightley that has most fans and bloggers working up a sweat. The scene is a turning point in the film. As Briony's eyes adjust to the gloom of the library, she witnesses Robbie pinning Cecelia against the bookshelves. With her emerald dress splayed against the dark wood, she seems almost suspended. Not one to kiss and tell, McAvoy diplomatically sidesteps any description of the scene other then to explain the mechanics of it: "There was a lip that was part of the library…there was a little shelf that came out and her bum kind of sat on that. I lifted her up and then it was all on from there."

And although he has described himself as a "non-alpha" male, McAvoy cements the perception of himself as a sexy leading man with this role. The star, who recently made People magazine's "15 Sexiest Men Alive" listing, will next appear in an adaptation of the Frank Miller graphic novel series Wanted directed by Russian action maestro Timur Bekmanbetov — this time opposite Angelina Jolie.