If I were writing a letter, I would always say "Mr."- or "Ms."-last name. Using both the first and last name after "Mr." or "Ms." seems too formal to me, but I also wouldn't feel comfortable using just the first name.
Meeting them in person would be more informal. But interestingly enough, I guess I subconsciously freak out about how to address the actors when I meet them because I can't remember ever calling any actor by name--last name, first name, title, anything. I just tell them what I liked about their performance, what other work I admire...but when I'm face to face with them I guess I really don't know what to call them.
I don't want to call them by their first name out of the blue like some crazy fan who thinks we're friends before we've met, but I also don't want to call them "Mr." or "Ms." like some crazy fan who feels like I'm not worthy to be in their presence (stupid, I know, because actors are greeted both of those ways all the time and I can't imagine they care either way). I just want to talk to actors like they're humans that I respect--but when I try to do that I end up avoiding their names altogether. Please tell me I'm not alone.
"If there is going to be a restoration fee, there should also be a Renaissance fee, a Middle Ages fee and a Dark Ages fee. Someone must have men in the back room making up names, euphemisms for profit."
(Emanuel Azenberg)