Bill Skarsgard. Let's just take a moment an thank the Skarsgard family for their beautiful and talented contributions to this world. Two of which are, ironically, played vampires. (In different series)
You know, this picture is really perfect. The Doctor is immediately interested in who Sherlock is. Sherlock is trying to deduce him, but having some trouble, given he is an alien and all. Clara is waving to John, trying to be friendly, and of course John picked out the girl of the group to pay attention to.