"She ended up winning. She gained about 15, 16 pounds. He gained 14," the film's director, Derek Cianfrance, told PEOPLE at Monday's Gotham Independent Film Awards sponsored by Euphoria Calvin Klein.
"Michelle was eating a pint of ice cream for breakfast and dinner and avocado sandwiches all day. She wanted to do it. She talked about her character having a certain self-hatred."
Blue Valentine is about a couple with a 5-year-old daughter whose marriage comes apart. Initially, Cianfrance envisioned Williams's character getting thinner as her emotional life unravels, but the actress, 30, had other ideas.
" 'I don't know about losing weight. What about if I gained some weight?' Cianfrance says Williams, 30, told him. He replied: "Surprise me."
And what was her ice cream of choice? "She ate that [Purely] Decadent stuff made with coconut milk," Cianfrance says. "Supposedly the coconut milk, the oil, has a lot of fat in it, and it's hard for your body to break it down."
Cianfrance also spoke about Williams's motherly instincts, which helped her in the role of Cindy. The actress's daughter Matilda, whom she had with Heath Ledger, turned 5 last month.
"Matilda came around [to the set], but not all the time, because she was in school," Cianfrance says. "But Michelle is such a great mother that she can get into the role of being a mother, and all of the caring and compassion that she has for children was apparent on camera."