Billy Crystal made a very successful career out of copying Stefan Gierasch's character here. This is no coincidence: Crystal - who already looked like Gierasch - willfully and deliberately modeled himself after Gierasch, to not only sound like him (voice inflections, speech pattern) but also personality-wise. Anthony Robbins used to sell an expensive cassette-tape course in the '80s, where he taught people to pick someone successful or whom they admired and to "model" themselves after said person. Billy Crystal set about to do just that with this character here. Some actors are lost and borrow another's personality and many have fashioned entire careers out of someone else's original and creative idea---and hard work.