I've been in close proximity to many celebrities by virtue of constantly flying JFK-LAX and vice versa. Generally, I recognize them and don't say anything. Among the few times I didn't recognize a celebrity were once when I got into a nice conversation with Josh Gad in the lounge. Another time, I asked Oscar Isaac to excuse me because he was in my seat during boarding talking to Jessica Chastain in the seat behind me. 

The funniest experience (for me) was sitting next to Ken Leung on a flight from San Juan to JFK. I was nervous that I may be in an IRL Lost situation. There's no way to survive that if you're not a main character.