When we first saw the news that David Tennant had left the cast of Burke And Hare, we thought it was pretty weird.
After all, Tennant's only at the start of his post-TARDIS film career, surely he shouldn't already be backing out of projects at the last minute.
Especially ones that sound as awesome as Burke And Hare. He was a title character (Hare) for Daleks' sake.
That is, unless the thing he's going on to do is guaranteed to be absolutely massive. Something he couldn't turn down. A career-maker. A Hobbit.
If you look closely at Simon Pegg's lovely (but sadly now deleted) Tweet that broke the story, the clues are there.
“David Tennant had to drop out of B&H but by sheer glorious serendipity a legend of Middle Earth and Skull Island has come aboard. Welcome AS "
So, Tennant has to drop out - presumably because he's been offered a role he can't turn down - and the Tweet that announces it contains both a reference to Middle Earth and confirmation that the person replacing him will be someone who has a far more flexible Hobbit filming schedule (Andy Serkis).
