Well, "say cheese" has a backstory. A school of thought - gives a logical reason - says that the “ch” sound causes one to position the teeth in a way, that when the long “ee” sound parts their lips, it forms something close to a smile.
UNSPLASH
The phrase was first used in the 1940s. According to Joseph E. Davies, former Ambassador to Franklin D. Roosevelt, it’s an instant formula for smiling when you're having your picture taken and it is certain to make you look pleasant, no matter what you’re thinking. Mr Davies revealed the formula while having one of his own pictures taken on the set of “Mission to Moscow.”