The sheer physicality of playing a character in chronic pain was the most challenging part of starring in the film "Cake," Jennifer Aniston said of a leading role that turns one of the world's most glamorous actresses into a bitter, unpleasant woman.

In the film, which had its world premiere at the Toronto International Film Festival on Monday, Aniston plays Claire Simmons, a woman suffering from constant pain and pill addiction who is haunted by hallucinations of a fellow support-group member who committed suicide.

To prepare for the role, the former "Friends" star drew on the experiences of two friends who lived with chronic pain. She also wore a brace that prevented her from slouching to mimic the stiff movement required.

"For me, it was really important to stay true to what the medicine I was taking physically would be doing to my body and at what point," Aniston said.

"It turns out if you walk like that for five weeks, you do actually become in pain."

As for the emotional aspect of portraying a woman dealing with chronic pain and grief, Aniston said she was able to go home at the end of the day and leave the psychological component at work.

"It was a hard place to be, but I slept and I was just eager to get to work every day. I really loved spending that time with (the character)," she said.

Much of the focus surrounding "Cake," has been on the performance from the 45-year-old Aniston, who went without makeup save for the character's white scars across her face.

An early review from Deadline called her performance an Oscar contender, while Variety said Aniston gives "a strong if self-consciously deglammed performance."

But the movie overall has received mixed reviews, with Britain's Guardian newspaper deeming it "about as fun to watch as sciatica."

"It's just the farthest thing from anything I've ever done," Aniston said of her decision to take the part in "Cake."

"I knew this would be an extraordinary challenge for me and I was so excited and so ready to dive in and take it on."

Aniston, who rose to fame starring on the hit television series "Friends" in the 1990s, has had a mixed record on the big screen, though she won acclaim for her roles in "The Good Girl," and "Friends With Money."

"Cake" is directed by Daniel Barnz, who came across Patrick Tobin's script while he was judging a screenwriting competition.

The script grew out of a short story Tobin had written after hearing the experiences of a friend who ran a support group for women in chronic pain.

At the screening, Barnz praised Aniston as being "one of the greatest actresses I know."

"Because the character herself can be difficult and acerbic and somewhat harsh, it felt like we had to cast somebody who you can forgive immediately," Barnz said.

(Editing by Jeffrey Hodgson, G Crosse)