Conversation so far:
Who were some of Gary Cooper's closest friends?
Ernest Hemingway was friends with Gary Cooper for 20 years.
What was important about Gary Cooper's friendship with Hemingway?
Hemingway believed Gary Cooper's character matched persona, once telling a friend, If you made up a character like Coop, nobody would believe it. He's just too good to be true.“

Question: Which other friends did he have?
Self-contained rewrite: Which other friends did Gary Cooper have besides Ernest Hemingway?