Michelle Williams says late ex-lover Heath Ledger was incredibly "vulnerable."

The former 'Dawson's Creek' actress - who has a two-year-old daughter Matilda with 'The Dark Knight' actor - claims his weaknesses were only revealed after his shock death from an accidental drug overdose in January.

In her first interview since Heath's death, she said: "I think the interesting thing about Heath, which maybe people have only really fully discovered in his death, is how vulnerable he was."

"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."

Michelle, 27, also spoke about Heath's battle with insomnia - a condition he was being treated for with prescription drugs, some of which were the cause of his eventual death.

She added to Interview magazine: "For as long as I'd known him, he had bouts with insomnia. He had too much energy. His mind was turning, turning, turning - always turning."

"But he had a talent for everything that he put his mind to. He didn't know limits."