Actors are much more likely to respond to a thoughtful comment on their work or congratulations on an award. The worst thing you can do is berate an actor for their character's actions. Some actors live in fear that an angry fan will attack them for something their character may have done. Separate the actor from the role before approaching them. If the occasion is very informal, relax and enjoy the company.
Celebrities lead very complicated lives while 'on the job', so vacations are taken very seriously.
Many famous people look forward to the opportunity to quit being famous for a short time. If you are fortunate enough to meet a celebrity on vacation, keep the atmosphere light and friendly. Discuss the beautiful weather or that fabulous restaurant in town. Suggest places to see or just grab a few drinks and relax. Don't get caught up in documenting the entire experience- live in the moment with your new friend who happens to live in California.