The 31-year-old says in the magazine's October issue that experiencing loss has changed her daily interactions, career and how she acts as a parent and friend.
The 28-year-old Ledger died in January 2008 after an accidental overdose of prescription drugs.
Williams plays another Hollywood star whose life was cut short, Marilyn Monroe, in the film, "My Week with Marilyn." It's about a week of Monroe's life in 1956.
Williams says she tried to gain weight for the role but it all went to her face. She opted instead to fill herself out with padding.
Williams says channeling Monroe made her feel "like being reborn."