"Maybe that's the secret," Williams said, laughing.
The enormous success of "Brokeback Mountain" catapulted both Williams and Ledger into super stardom.
The couple had their daughter, Matilda, in 2005, but the two were separated by the following year. Then in January 2008, Ledger was found dead in his New York City apartment from an accidental drug overdose of prescription medication. He had just wrapped up shooting the blockbuster hit, "The Dark Knight."
Williams' relationship with Ledger is a subject she has declined to discuss, but said she said she "understood" people's curiosity about how she has coped with his death.
After he died, Williams said she was comforted by reading "The Year of Magical Thinking" by Joan Didion, a book written about Didion's difficulty in letting go of her husband, John Gregory Dunne, in the first year after he died. Williams said she experienced very much the same thing.
"It didn't seem unlikely to me that he could walk through a door, or could appear behind a bush," she said. "In some ways, I am just, I am sad to be moving further and further away from it."
Like Didion who writes about how life can change in an instant, Williams said she, "got kind of obsessed with that for a while, [the] 'before and after."
Now raising 5-year-old Matilda, Williams is a dedicated and enthusiastic mother. She said she wants her daughter to have a normal life, but acknowledged the requirements of her career makes that a "challenge."
"It is of more importance to me than anything else in my life," she said. "I would re-arrange anything to make that possible. If something starts to encroach on that ... it's gonna be removed from the equation."
Despite the sorrow in her life, Williams is clearly mending and moving forward.
"If I could go back and tell myself at 15 that I would be allowed to make this kind of work and I would be in this kind of community, I wish I could tell that girl that everything was going to turn out okay in her work," Williams said. "It's strange, at every point I was like, 'that's enough for me' and then I'd get given a little bit more."