@OnTheBonnyBanks Ooh are you famous?
It's a difficult one I think, I certainly wouldn't interrupt somebody's meal or be a nuisance if they were with family but I have to admit to asking for autographs on occasion. I did approach Frank Bruno years ago at Gatwick airport and asked for an autograph but I was a child and he was very lovely and jolly. Did the same when I saw a coach containing the band Heaven 17 at a service station. I was a big fan and couldn't believe my luck (I was about 13!). Again they were really nice and happy to oblige. I wouldn't do it now though.
I suppose being approached comes with the territory. It was the rude and entitled I took issue with. No "excuse me or apologies for interrupting", just "are you so and so, sign this".