Re celebrities being a bit grumpy when off duty and someone approaches them: they can’t be ‘on’ all the time, can they? They are still human.
(I am laughing at ‘fuck you Gandalf’!)
Whoops, I may have inadvertently repositioned Ian McKellen as a taxi-stealing celebrity villain. 
It's actually people's behaviour around famous people that interests me. I was once in some glossy Knightsbridge hotel lobby waiting for a friend and Robert de Niro and Al Pacino walked in, and I didn't clock them for a bit they were just two important-looking, well-groomed middle-aged men in suits and the place was full of similar, and I was tired, late for something and annoyed with friend but I swear, there was a change in the air, as other people responded to Bigtime Fame entering the building, even staff trained to deal with the famous with discretion, and insanely wealthy and powerful fellow-guests.
What struck me too is that as well as having a couple of giant security guards, neither of them ever looked around them, which you'd probably train yourself not to do if everywhere you ever went you were the person everyone was looking at. I think that that was why I first noticed them -- everyone else was looking at them, or pretending not to, and they weren't looking anywhere but at one another or the floor.
I assume they were promoting a film, but the only thing I remember about when it was that I'd just come from the Hyde Park Last Night of the Proms prom where Sue Perkins conducted something after she won Maestro.
And, not unrelatedly, the friend I was waiting for worked for a classical music agent, and was upstairs dropping off dry cleaning or something for an opera singer who was in town for the Last Night. At around 11 o'clock at night.