Heya folks, newbie here (be gentle). I must say, I have been thoroughly entertained by your experiences with encountering The Famouses (and I'm not surprised Antony Cotton was constantly cropping up in the unpleasant category - I've always disliked him).
If anybody is interested, I'd like to contribute a few of my encounters:
Michael Palin: I went to a book signing of his and I recall being exceedingly shy and even letting people go in front of me in the queue I was that terrified of approaching him (I am a huge Python fan and Palin is my favourite so it made it 10x worse). Eventually I came up to him but I can't remember an awful lot because I was in a daze the entire time I was in his presence but he was very nice (shock horror) and patient with me. I got two signed books and a photo, so all in all it was a success.
Charlie Edwards (Gregson from
Downton Abbey, Lady Edith's ex-lover): Met him on numerous occasions. Arguably the nicest individual you could ever meet: Unassuming, amicable, down to earth, not to mention he is literally Palin's doppelgänger. Has time for everyone, signing programmes, getting selfies, etc, after his theatre performances, and is a pretty good hugger

He's recognised me a couple times (I am a big fan of his, too) and knows my name, so, yeah, he's awesome
Martin Clunes: Saw him at the BFI Southbank after a screening of
Arthur & George. We exchanged maybe 3 words but he seemed jolly nice and he let me grab a quick pic with him. Despite the fact he was being rushed off to the green room by an impatient bouncer-type bloke, he took the time to speak with fans that day, albeit briefly.
Hattie Morahan: We had a little chat and she was very pleasant, but was in a rush to meet her family so she had to end our convo early, but it was totally fine.
Anne Marie Duff: One time she asked if she could borrow my sharpie to sign for a fan (and I am constantly kicking myself I didn't get her autograph too when I had the chance).
Ian McNiece (Bert from
Doc Martin): Another nice, very charming individual. He's just a big ol' teddy bear.
Jemima Rooper: Really sweet, but really tiny, bless her.
I've never had any nasty encounters, thankfully, but I will note
Steve Thompson (TV writer, written for Doctor Who and Sherlock, amongst others) was a bit aloof when I got his autograph. He didn't say anything, just, rather reluctantly, signed my copy of a play he'd written and walked off. He was with his children if I recall, so maybe he was rushing off with them, I don't know, but I wasn't too fussed.