i always wonder what i'd do in a similar position. i imagine someone like Terrance Dicks (you won't know who that is) just walking into somewhere like that and i have no idea what i'd do. i guess just going up and asking them is a good thing, but make sure you have a pen and paper handy so you can ask them for an autograph if it actually is them; it would look very weird if they say yes and you just walk away. not that a porn star's autograph is something you particularly want; it just gives you a reason to be in their face and not feel awkward over.