Both are former SOUTH HAMPSTEAD HIGH SCHOOL FOR GIRLS students, and, even though they didn't know each other there, they share the same history.
Williams, who admits she was a "pretty insignificant nobody" at school, recalls FIGHT CLUB star Helena was always destined for greatness.
She says, "She was an extraordinary person who had a sort of cult following.
"Meeting up again and playing sisters was ideal because we didn't have to invent that common history. We had the same humour and we remembered the same nightmare history teacher, and the uniform we had to wear.
"The intervening Hollywood years disappeared and we were back to being Hampstead school girls."