NEW YORK -- Before you can get to Michelle Williams' dressing room at the Belasco Theatre on Broadway, you pass one with a sofa upholstered in bright colours, a flat-screen TV and a disco ball attached to the ceiling.

It's reserved for her 10-year-old daughter, Matilda, who can now come to work with mom and hang out, knitting or watching movies. There's one rule: She must not know what mom is doing down on the stage.

Williams is starring opposite Jeff Daniels in "Blackbird," a disturbing tale of an older man, a much younger woman and what happens when they meet 15 years after their brief and illegal relationship has ended.

"She can't see the play, be anywhere near the play, hear the play, listen to the play," Williams said. "This is so she can come and hang out with mom, but with a lock on the door."

Downstairs, Williams plays a vengeful victim, hurling accusations at her former lover with devastating accuracy, and yet harbouring a few problems of her own.

The actress said the topic is, unfortunately, always timely. "You can't not know somebody who's been abused. It's far more common than it should be," she said. "So I find it very specific and very general."

Williams, who on this day apologizes for needing to wolf down rice pudding in her dressing room before her show, seems to have found a way to balance work with single motherhood. She is raising her daughter with the late Heath Ledger in an apartment in a charming, cobble-stoned neighbourhood of Brooklyn, above a grocery store.

To accommodate Williams' schedule, producers have agreed to have no shows on Sunday -- traditionally a key box-office draw -- so "we can have a real life." The two recently attended a concert by the girl's favourite singer, Stevie Wonder, and she shows a photo on her phone of Matilda wearing an ecstatic expression. "She's loved this life as much as I have," Williams said.

David Harrower's "Blackbird" marks the first big job Williams has tackled since she spent the better part of a year singing and dancing in the latest revival of "Cabaret" as the complex nightclub singer Sally Bowles.

"That took the stuffing out of me," she said. "I put myself out to pasture for a while after 'Cabaret.' I ate a lot, literally, and ran lazily around fields because I was so tired. And then this came up and I just couldn't say no."

Williams' film career took off with 2005's "Brokeback Mountain." She received the first of her three Oscar nominations for her performance as the rejected wife of former real-life partner Ledger's cowboy. Her others came in "Blue Valentine" and "My Week with Marilyn."

Although she had done some theatre, she decided to make her Broadway debut in "Cabaret" in 2014, wearing lingerie and belting out "Don't Tell Mama" and "Maybe This Time" for 10 months.

"I felt like I was pushed up against the edge of my ability every night. But, as a result, I feel like I could feel my ability just inch up," she said. "So that's been exciting to go back to work with some new ideas and abilities."

Joe Mantello, the Tony Award-winning director who helms "Blackbird," said Williams, 35, is essentially a very curious person: "I think there's a fearlessness about her. I'm not surprised that she's pushing herself to stretch in new and exciting ways. It just seems to be part of who she is."

When she was offered "Blackbird," she was immediately impressed by its beauty and consulted with her friend and "Cabaret" co-star Linda Emond, who called it one of the most exciting pieces of theatre she'd seen.

So, after doing two small parts in two independent films -- Kenneth Lonergan's "Manchester by the Sea" with Casey Affleck and Kyle Chandler, and Kelly Reichardt's "Certain Women" with Kristen Stewart and Laura Dern -- Williams went back to the stage.

"It's sort of like having a second child," she said. "I learned so much in those 10 months that it would be a shame to give up all of that hard-won knowledge."