Nearly 20 years after having a child with Heath Ledger, Michelle Williams has paid tribute to the Australian actor, saying he was "so special".

Michelle Williams has paid tribute to her former partner Heath Ledger as "so special".

The 44-year-old actress had daughter Matilda, now 19, with Ledger before he died in 2008, aged 28, from an accidental overdose as a result of prescription drug abuse, just months after the couple separated.

She has now spoken candidly about the Australian actor and her approach to motherhood during an appearance on the Armchair Expert with Dax Shepard podcast, saying "(he was) so special, so special. Thank God there's Matilda".

Williams also became emotional as Shepard recalled his own connection to Ledger.

"I feel obligated to say that I knew him a little bit when he was getting sober, and I don't know that I've ever fallen in love with somebody so quickly," he said.

"This is one of the most special boys I've ever met, and I can feel the weight of the world on him in a very special way that kind of broke my heart. I was very, very sad, and I thought he was just so special."

Williams and Ledger met while filming Ang Lee's 2005 film Brokeback Mountain, which earned widespread acclaim and multiple Academy Awards.

During the film's promotion, the couple learned they were expecting their first child and welcomed Matilda in October 2005.

"We had a baby. But I suppose maybe it's a good thing about being young is that you don't have so much life experience that you can contextualise things," Williams said.

"So you're really just going with the flow."

Williams, who now has three more children with her partner, director Thomas Kail, spoke about the challenges of balancing parenthood with her career.

"Kids are such great life checkers, they force you to put your best self in front of them," she said.

With AAP.