Personally I think that if you see them on the street, or just generally out and about, 'celebrities' are under no obligation to even talk to you. As long as your approach to them is polite, I think they can achieve this quite easily with "sorry this isn't a good time", or something along those lines. Anything more than that and it's a bonus. I don't think actors/musicians/celebrities owe people anything in that context.
Obviously if you approach them when they are engaged in something else, or they are spending precious time with their family, you're being a bit intrusive anyway.
However, I think if they are attending an event as a guest or promoting something and being paid, they should probably try to meet a minimum standard for 'pleasantness'. It's always good to have a handler to take the flak for moving people along quickly etc. rather coming across badly.
If it's a paid gig then they should bring their Pro Game. I've only really met two 'celebs' and they were both Pros.
The first was UK TV presenter Jonathan Ross. He was at the Birmingham Comic Book Convention promoting his new comic book and he was out working the crowd and being a nice guy to anyone who approached him. His handler was on tap to keep him moving, if he got bogged down for too long. But he also ignored his handler from time to time to take extra time with a few people. I managed to get this photo (I'm not in it but it was promo for my fan film)
Second was Karl Urban, whilst he was doing promo for Dredd at a London Expo. I think I read somewhere that he takes his promo commitments very seriously and that it was something he learnt from Viggo Mortensen. The event had strict rules for photos etc. I had arrived with my son in full Dredd costume. He took time to compliment my son's costume. There was a very subtle glance between himself and his handlers and the rules were bent for a quick photo. Considering this was at the end of a two day convention I was very impressed with Karl. Personally I don't think I could have lifted my head off the table by that time, but Karl was still happily chatting to people. I know quite a few Dredd fans who also attended the event and they all had a little story to tell about how Karl made them feel special and how the PR team for Dredd took time to engage them.
Another thing I love is when actors turn up in costume. Not only does it keep a layer of personal privacy, it also allows the public a chance to see the character they love, rather than the actor, who is often not as interesting. It also shows the public that the actor is getting into the fun of the event. I was at an event in Birmingham and two of the actors from UK Sitcom 'Allo 'Allo!' had come in costume. I'm not really a fan of the show, but I had a sudden urge to get a photo with them, which I'd never done before. I think Robert Maschio from 'Scrubs' does something similar.
