Before David Tennant was the tenth Doctor he was involved in the recording of several Big Finish Audio stories. In the fall of 2003 Nick Briggs, better know as the voice of The Daleks, was getting ready for the third installment of his Dalek Empire Series. He ran into David at Mark Gatiss' birthday party and David asked if there was a part for him. You can click here and listen to Nick tell this very funny story. In the excerpt you can here David say he was just finishing a play at the National and they had asked him to do another one, but that he rather be involved in the Doctor Who project! If you want to know more about David's appearance as Galanar click here.
The play he is talking about is Pillowman, and I speculate the play he turned down was Measure for Measure. (IMHO)