As a break from portraying the Hulk in the infinite "Avengers" franchise, Mark Ruffalo took on another character with fearsomely mercurial moods. Ruffalo takes on the role of Cam Stuart, seen here in a scene from the film, "Infinitely Polar Bear." THE CANADIAN PRESS/AP/HO - Sony Pictures Classics, Claire Folger

TORONTO - As a break from portraying the Hulk in the infinite "Avengers" franchise, Mark Ruffalo took on another character with fearsomely mercurial moods.

In "Infinitely Polar Bear," the 47-year-old Ruffalo plays Cam Stuart, a charming eccentric raising two daughters while coping, barely, with bipolar disorder.

Much of the story is autobiography for first-time writer-director Maya Forbes, who helped Ruffalo inhabit her late father's idiosyncratic mannerisms by passing on his old Super 8 home videos, letters and photographs.

Ultimately, Ruffalo absorbed her dad's self-styled accent, his chain-smoking and even his oddly regal gait.

It was a little eerie for all involved, honestly.

"There was a lot of built-in pitfalls," Ruffalo conceded during September's Toronto International Film Festival.

"It feels like a big responsibility and you want to honour him. It was a constant sort of measuring of what I was doing against what Maya thought it should be."

"It's sort of like having a walking photo album that talks," Forbes said in a separate interview. "It's pretty crazy. Anyone who knew my father is just like: 'I cannot believe that he's up there like that.'"

To put her dad up there, Ruffalo had to put himself out there, with a daring performance that edges into downright uncomfortable territory at times. (Not to mention all the smoking, which Ruffalo said left him bed-ridden for two weeks with bronchitis after the shoot, "sicker than (he's) ever been.")

He makes his onscreen introduction clad in minuscule red briefs, tearing around his family on a bicycle during a particularly ugly episode, while the children's mother, Maggie — played with limitless patience by Zoe Saldana — tries to escort them out of harm's way.

Other times, he's all freewheeling whimsy, a source of spontaneity and joy for the girls he oversees by himself in Boston while Maggie pursues her business degree in New York.

Even as Cam's volatile behaviour veers toward the seemingly dangerous, the movie bounces along at an upbeat pace — a relief for Ruffalo, who wanted to avoid anything "too maudlin or too precious."

"Bipolar people are, half the time, a lot of fun," said the two-time Oscar nominee.

"People I know who are bipolar are probably the most charming, funniest, smartest, most driven people I know — except for when they're not."

Forbes, who cast her own daughter as the onscreen version of herself, lost sleep wondering whether she'd calibrated the character just right.

"I had this total crash and panic about, 'Oh my God, people are not going to like Cam. He's so annoying and what have I done? I've done a horrible disservice to my father, whom I loved,'" she recalled of the film's first screening.

"My sister said: 'Well, not everyone liked dad in real life all the time.'"

For what it's worth, Ruffalo knows where he stands.

"By any standard, he's probably the most unorthodox parent you could possibly have," he said. "But his love for them is so deep. His commitment to them and his belief in them.

"His style of parenting — if somebody doesn't get really hurt or damaged along the way — is pretty powerful to create people who are well-possessed and capable and fearless.

"When I look at Maya and (her sister) China, who are the byproduct of Cam and their mom, I'm pretty damn impressed. They're great girls."

"Infinitely Polar Bear" opens Friday in Toronto, Vancouver and Montreal before expanding to other Canadian cities.

Follow @CP_Patch on Twitter.

News from © The Canadian Press, 2015