Well, it happened somewhere between "Thin Ice" and "Knock, Knock", that much I know :D Capaldi was always the Doctor I'd liked most (before him, the Doctor was usually my least favourite character in any given season), "Heaven Sent" made him into my favourite character, and I enjoyed S10 and Bill and Twelve's relationship from the start, but I don't know what happened in "Thin Ice" to suddenly make me fannish