Ledger, who is the father of Williams' 2-year-old daughter Matilda, died Jan. 22 of an accidental lethal combination of prescription drugs. He was 28.
Williams has been keeping a low profile since his death, but spoke candidly in an article for the April issue of Interview, Usmagazine.com said Thursday.
"I think the interesting thing about Heath, which maybe people have only really fully discovered in his death, is how vulnerable he was," Williams told Interview. "You can pick it up on it in his performances, but it's easy to overlook because he was so physical and beautiful and strong and masculine. But there was always that underlying sensitivity. That's who he was."
Williams also confirmed reports the actor had experienced trouble sleeping.
"For as long as I'd known him, he had bouts with insomnia," she said. "He had too much energy. His mind was turning, turning turning -- always turning."
"He had a talent for everything that he put his mind to. He didn't know limits," she added.