Starting with the most obvious. As you all are undoubtedly well aware of, David Tennant embodies the ever popular tenth incarnation of our beloved Doctor. In Harry Potter, however, he portrayed a less popular character called Barty Crouch Jr who was, amongst other unloveable features, responsible for the torture of the Aurors Alice and Frank Longbottom, parents of Neville Longbottom, one of Harry's most loyal friends. Fun fact: The first chapter in Harry Potter and the Goblet of Fire to mention Barty Crouch Jr is titled 'The Parting of the Ways' which, coincidentally, is also the Doctor Who episode in which the Ninth Doctor Christopher Eccleston regenerated into David Tennant's incarnation. And that's not all! The Goblet of Fire is also the first book/film in which fans were introduced to 'bigger on the inside' technology (the tents at the Quiddich World Cup as well as Barty Crouch Jr's chest). Everything's coming up Doctor.