If I am speaking respectfully to someone, I would say "Good Morning, Mr Williams" or "It's a pleasure to meet you, Mr Steinbeck - and can I say how well you look for a dead person!".
If I were announcing someone formally at a ball, I might say "Mr John Williams" as he came into the room. If I were calling for someone in a waiting room at the doctors, I would say "Mr Williams" or "John W