Conversation so far:
what is the movie it's a wonderful life about
It's a Wonderful Life is about angel sent from Heaven to help a desperately frustrated businessman by showing him what life would have been like if he had never existed.
who directed it's a wonderful life
It's a Wonderful Life was produced and directed by Frank Capra
what is the movie it's a wonderful life based on
It's a Wonderful Life was based on the short story and booklet The Greatest Gift, which Philip Van Doren Stern wrote in 1939 and published privately in 1943.

Question: who wrote the screenplay
Self-contained rewrite: who wrote the screenplay for it's a wonderful life