Keep a Straight Face: Keep a serious expression and demeanor when you have the urge to laugh.
Charlie struggled to keep a straight face when Marie said she had seen a real UFO.
This saying usually refers to concealing the urge to laugh, but it also extends to concealing any emotion. It implies that laughing or being overly emotional would be inappropriate in the current situation. It can also be used if someone doesn’t want to tell a secret or expel personal information in a particular situation.