The actress shares several intimate moments with Ryan Gosling in the upcoming drama, which she has been busy promoting at the film event in Utah.
But when it came time to sit down and watch the film, Williams kept out of the cinema for the sake of Matilda, her four-year-old daughter with the late Heath Ledger.
She tells MTV.com, "When the movie screened the other day, me and the kid, neither of us were old enough to watch that movie. So we coloured in Tinker Bell pictures for two hours and played pirate games."
Gosling admits he was also nervous at the Sundance screening - because he brought his mum along as his date.
He adds, "It's a racy movie, so it's hard to watch those scenes with so many people. I watched it with my mother, she was beside me. I gave her an iPod. And when the naughty scenes came on, I pressed 'play' and covered her eyes."