Author John Irving's mother was not married at the time he was conceived. He never met his father nor would his mother talk about him. He eventually told her that if she didn't tell him about the father he would invent the man and circumstances of his conception. Her reply was "Go ahead, dear." "The World According to Garp" was that result.
John Irving recalls how funny and profane Williams was while being waxed for the role of young Garp in his novel "My Movie Business".
Glenn Close plays Robin Williams's mother, yet she is only four years older than him. Similarly, Close and actress Mary Beth Hurt played women from successive generations yet in real life they are only one year apart in age.