I just watched Imagine Me & You and the actor who played Giles was in it and while I recognized him immediately, he has some chameleon properties for sure. Plus in Merlin it appears he's playing against type, so there are just all sorts of excuses for you! One it took me a long time to get was Leighton Meester in Gossip Girl. It took me most of the first season to realize she also played Carrie Bishop on Veronica Mars.