David was brilliant. Took my four year old son to meet him as he loves Doctor Who especially the tenth and eleven doctor.
It just so happened that we bought him a figure from a dealer that he had been after for ages of Matt Smith in his regeneration outfit. From a distance of course it looks like David Tennant's doctor. When we walk up to the desk to have the photo signed, David said to him 'Hi, how are you, what have you got there' thinking it was his figure.
Of course my son proudly told him 'It's Matt Smith!' which caused everyone including David to laugh.