All I did was awkwardly stare at him.

I was in an elevator waiting for the doors to shut when some generally cheery guy walks in and says "Hey man, hows it going!" I answered and realized I was standing in an elevator with Paul Rudd. I immediately second guessed myself. "Theres no way thats Paul Rudd." Instead of saying hi, I just kind of stared at him while the elevator was going down. When we reached the ground floor he turned around and said "Have a good one!" It was Paul Rudd and all I did was awkwardly stare at him. I also once saw Carrot Top in the Phoenix airport. I confirmed it was Carrot top because he was wearing rainbow socks.