TORONTO - Diving into a film about the tumultuous teen years of a bright but rebellious John Lennon was "suffocating" at times, says director Sam Taylor-Wood.

The British photographer turned movie director says she naively embraced a script for "Nowhere Boy," which chronicles the legendary singer's life before the Beatles. It was only as she began filming that she fully realized the enormity of the subject matter, and was barraged with advice and warnings over how to portray -- and not portray -- the icon.

"It was stupidly undaunting at first because I went into it so radically naively and just thought, 'Wow what a great story, I'd love to tell this story,"' Taylor-Wood says during a recent stop in Toronto to promote the film.

"Not until I'd stepped off the train in Liverpool did I realize the enormity of the subject matter I was taking on. Everyone took pride in telling me how big a subject it was and how I musn't mess up... 'Don't do this. Do do that,' and then more and more people (kept) coming out of the woodwork. There were times when it was really suffocating, actually, having this subject matter, but at the same time, you know, I found that I sort of loved Lennon more and more as I kind of got to know him."

"Kick-Ass" star Aaron Johnson plays the budding musical genius, depicted here as a defiant 15-year-old who clashes with all authority, especially his straight-laced guardian Aunt Mimi, played by Kristin Scott Thomas.

He yearns to find his birth mother, and one day learns from a cousin that she's secretly been hovering nearby the whole time -- living just around the corner with another family. Anne-Marie Duff plays the vivacious but troubled Julia, a woman haunted by her past and desperate to reassemble her splintered family. During a brief reunion she introduces Lennon to rock 'n' roll, buys him a guitar and teaches him to play.

Johnson say he knew nothing about Lennon's youth before working on the film, and strove to present a figure that was "deeper than the showman."

"This is a side that's never been seen before -- this is his insecure, vulnerable side, his side that he's put away for a while -- the pain and anger," says Johnson, 20, who auditioned for the role while shooting "Kick-Ass" in Toronto and Hamilton.

"I took two, three months studying him, intensively, like from a music element, from the rock 'n' roll side of it and who his inspirations were.... I read biographies, I picked out details, I watched all the documentaries, I listened to video recordings and tapings of him."

Invaluable input came from Yoko Ono and Paul McCartney, says the 43-year-old Taylor-Wood, noting Ono in particular sparked a rewrite when it came to their portrayal of the tough-minded Mimi.

She says Ono urged her to avoid vilifying the strict aunt or turning her into a "dragon-like figure, which a lot of biographies have done."

"Yoko said, 'You really have to understand that the relationship with John and Aunt Mimi was much more powerful' and so we rewrote a lot of that," says Taylor-Wood, who made her name as a fine art photographer with collections that include unguarded celebrities crying or sleeping.

"(Mimi) started out as much more formidable and then as we worked with her we kind of realized that she was a mother who brought him up and so it wasn't just so straightforward."

McCartney offered up minute details about music equipment and mannerisms, revealing that even as a young man, Lennon had such bad eyesight he could barely make out figures in the distance.

"John would think he could see people on the street but actually it was like a tree or something like that -- he was that shortsighted," says Taylor-Wood, whose own mother left the family when Taylor-Wood was 15.

"That kind of gave me a clue into that whole set of Mimi always telling him to put his glasses on because I didn't want him sort of constantly squinting throughout the film and so it was just a sort of reminder of little things like that which were really important."

Getting the details right was a challenge, even with all the material that has already been written about the singer, artist and activist, she says. Taylor-Wood notes that screenwriter Matt Greenhalgh spent years researching Lennon's early years and continually came across inconsistencies.

"Trying to find and fathom a truth within all of that was difficult for him but I think as close as he could he got to whatever truth (there was) and most of what we could get we got from people who knew him."

Finding the right actor to embody the burgeoning star was crucial. Taylor-Wood praises Johnson with bringing an intensity and maturity unmatched by nearly 300 other actors that auditioned for the role.

"He'd been playing 'Kick-Ass' for Matthew Vaughn and so he came in straight from that set and I thought he'd still be the sort of nerdy American kid kind of state but he came in as Lennon," says Taylor-Wood, noting he came in relatively early in their search.

Johnson and Taylor-Wood sparked a tabloid firestorm across the pond when they went public with their May-December romantic relationship, which Taylor-Wood insists developed after production ended.

"There wasn't flirting but I think there was a sort of, I don't know, just something I'd not felt before with anyone which was that sort of innate understanding," says Taylor-Wood, who conducted interviews between nursing the couple's newborn baby.

"It kind of came from that sort of, I don't know, some sort of higher spiritual aspect, more than anything else."

"Nowhere Boy" opens Friday in Toronto, Vancouver and Montreal.