My dad is a conductor so has met a few celebrities (almost enirely musicians or presenters) during shows, some who've been very nice and others who haven't.
Andrea Bocelli was late for every rehearsal, offered no apologies for it and refused to sign autographs for anyone in the choir. Paul McCartney bought everyone a present after they did their show but wouldn't let anyone come near him and had my dad vetted by about 4 different security guards before he was allowed to speak to him. Graham Norton & Rufus Wainwright were lovely and very charming, posed for photos and signed autographs for everyone. Nick from The Apprentice is the nicest person he's ever met though - was in the audience and came up to speak to my dad afterwards and said what a good show it was, asked how long he'd been conducting, bought him a drink in the bar etc.
A friend of mine went to school with George Ezra and said that he was really unpleasant back then and acted like he was better than everyone else because he was in a band.
Personally, I met a currently popular tv chef this time last year at a function he was making an appearance at. I went over to talk to him because I love the show he's on and he was absolutely charming initially, but then he asked me if I wanted to go for a drink after and got quite arsey with me when I politely turned him down since he's old enough to be my dad (something along the lines of "I could have any woman here if I wanted to" was said).
Also met Emma Bunton in the late 90s and remember her being very down to earth and polite.