The Casanova star, who takes the TARDIS for a spin for the first time on Christmas Day, told tonight's Jonathan Ross Show: "It's a bit of a gamble. The danger is that you get stuck with it, but the other option is you don't do it. I didn't want to be that guy. As long as I don't get the sack, I'll do another series."
Meanwhile, Tennant admitted apprehension over the audience's reaction to his Doctor. "It's almost too exciting, all a bit daunting. It was the most successful TV show of the year; if it all goes wrong it's my fault."
