Matt Smith, the actor who took over the role of the Doctor on Doctor Who after David Tennant’s run, also auditioned for the role of Watson on the BBC series Sherlock. However, the show’s creator Stephen Moffat thought Smith’s manic style better suited Holmes, but the part had already been cast to Benedict Cumberbatch. Interestingly enough, Cumberbatch, himself a fan of Who, talked with David Tennant about taking over the role when he left, but Stephen Moffat instead cast that role to the aforementioned manic Matt Smith.
I’m not sure if there’s any animosity between Smith and Cumberbatch (and I doubt there is, given that they both seem like agreeable chaps), but the same cannot be said for the Doctor and Sherlock. Behold the awesomeness that is Anything Sherlock Can Do, The Doctor Can Do Better:
And while we’re on the topic, here’s Tennant’s Doctor and Smith’s Doctor doing the same thing:
And, yes, you will have that song stuck in your head the rest of the day.