Yeah, DT is lovely, but Christopher just had this presence to him. I have to admit, a lot of it is in the way that Jack reacts to the different regenerations.

Jack just submits to the Ninth Doctor in this completely gorgeous way. You know he's still Jack, still a flirt, still cheeky and adorable and challenging, but he just gives himself over to the Nonth Doctor in a way that he doesn't quite do with the Tenth.