Michelle Williams was in character while portraying Marilyn Monroe "at all times."

The 31-year-old actress personates the Hollywood icon in newly released biographical drama My Week with Marilyn.

The film directed by Simon Curtis chronicles the time Marilyn spent being escorted around Britain by young aspiring filmmaker Colin Clark in 1956 shortly after the star wed famed playwright Arthur Miller.

Michelle channeled Marilyn's character unremittingly while filming.

"I tried to keep her voice, I tried to keep her walk, and I tried to keep as much close by as I could at all times," Michelle told Fox News at the film's Los Angeles premiere.

Michelle feels honoured to have had the opportunity to play such a cinematic paragon.

"[People think] that maybe she wasn't as bright as she actually was," she explained. "I just feel so lucky I never thought I would wind up here, playing Marilyn."

Director Simon is grateful for Michelle's powerful commitment to the role.

"Michelle's a great actress playing a great icon. I was just desperate for her to say yes' [to the part]," Simon recalled. "She was intrigued; she liked it, but had to go through her own process, which I completely understood, because it was a massive thing to take on But when I finally saw her done up as Marilyn, it was a very exciting moment. The journey of watching her performance evolve on the set, day in and day out, was the most exciting moment of my career."

My Week with Marilyn is currently in theatres.