You get a lot who aren't so recognisable off camera because of make-up, clothes etc. Like you'll get the ultra smart boy bander who dresses grungy, doesn't shave, wears a cap in real life to get around. I know one big TV presenter wears a scarf across his mouth, lol. It depends how distinctive they are and where they are. If people know they are going to be somewhere (an event etc) then they will be more upfront about approaching them.
It can be a bit of a crap life. It balances out - on the one hand you get invites to everything and lots of attention - on the other hand you don't know who to trust or who wants something.
For the reality z-listers its a bit more scary because you get thrown into it, carried away with it, then once your 15 minutes is over people will still recognise you to an extent day to day, so good luck with going to tescos and buying up those reduced goods, going and signing on, or the worst, going out clubbing and every drunken yob wants to shout insults or attack you cos you "used to be off the telly".
I agree with Lexis rule of thumb.