Directed by Sarah Polley (Away From Her), the indie comedy-drama stars Williams as a woman caught between two men - Seth Rogen's loving husband and Luke Kirby's alluring bohemian neighbour.
"I gave the script to a friend of mine to read," Williams told The Hollywood Reporter. "I was like, 'I'm so excited, I'm going to make a comedy!'
"And I came back an hour later and she was on the couch reading it and she was crying, and she was like, 'I don't know when it's going to get funny, Michelle!'"
Williams added wryly: "Yeah, that's my kind of comedy..."
She discussed the fact that the film centres on a transitional period of life to which she could relate.
"I think one of the most important things for me is that I thought it was talking about a transition that you make in your life in your late 20s from being a girl into being a woman, and that's the transition that I was in."
Take This Waltz premiered at last year's Toronto Film Festival, and will open in US theatres on June 29. Its UK release date is yet to be confirmed.
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Watch the trailer for Take This Waltz below: