TORONTO: 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.

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.”

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.”