Yeah I know what you mean. I met Ewan McGregor in my local pub (can you believe that?) and I think I was probably a bit goofy at first. I was about to ask "Aren't you...?" (I know - dumb!) and anticipating the question he laughed and said "Aye, it is me." I guess he gets that rather a lot.
Having said that, I had been there a while and had several pints of liquid comedy, so the bravado kicked in and I asked him for the photo/autograph thing - which he was kind enough to do. I now have a great shot of us blowing kisses to the camera together.
He's a really nice bloke actually. We chatted for about an hour or so, then he bought me a pint and left.