You don't. If you know Bill Milner, his actual name is William but in The Son Of Rambow he done it with someone else called William (but they call him Will) and they don't want to get mixed up! So Bill Milner has been named 'Bill' for his career... I think....
He didn't "change" his name, Bill is the familiar version of William. His name is William, but he goes by Bill.
His name still is William Gates, "Bill" is a common nickname for William.
Bill Bill
Bill Paxton's birth name is William Paxton.
Usually it is William.
Bill is short for William do perhaps "Super William".
Bill comes from the name William, which means "protector."
William Bill Bailey Rose
William
Bill Goldberg's real name is William Scott " Bill " Goldberg.
Bill Cowher's full name is William Laird "Bill" Cowher.
William Jefferson Clinton Clinton is his adopted name but is his legal and binding name.