In the new book she has co-authored with her mum Helen, the Desperate Housewives star has revealed she had an unwanted sexual experience with the 18-year-old daughter of a family she visited when she was just 10 years old.
And her shocked mum admits she never knew about the incident until she read her daughter's confession before it appeared in the memoir You Have No Idea.
Williams wrote, "Family friends who had a daughter invited me to visit their friends in California with them. The family we stayed with had two kids. Susan, who was 18, smoked, drove a car, and was the epitome of cool. (My friend) Nancy and I slept in the den. One night Susan crept in. She told me to lie down on the rug. I was confused. Are we going to play a game? As I tried to make sense of it, Susan pulled down the bloomers of my cotton baby-doll pyjamas.
"'What are you doing?' I asked. 'Don't worry - it'll feel good.' I lay there paralyzed. What was going on? I didn't speak. She kept at (the molestation) for I don't know how long. She slid my bloomers back up and whispered: 'Don't tell anyone.' I climbed into my bed, trying to figure out what had just happened.
"For years I kept Susan's visit to myself. I didn't really understand until college. I was with with my boyfriend and it hit me and I blurted it out: 'Oh my god - I was molested!'"
Appearing alongside the singer/actress on U.S. news show Access Hollywood Live on Friday (20Apr12), Helen admits she's glad she didn't know about her daughter's first sexual encounter.
She says, "I probably would have been a very unhappy, angry mother and I'm not sure I'd be wanting, at this time in my life, to be in jail for somebody doing something... that I would have had to do some bodily harm (to)."