It looks like something has tickled David Tennant in this picture taken during location filming on Series 4's premiere episode Partners in Crime which featured the timely return of Donna Noble. It's no wonder he's finding it hard to control the giggles with Catherine Tate, comedienne extraordinaire, around. If you've watched the cast video diaries on the Series 4 boxset you'll know all too well that there was plenty of fun to be had with these two in the TARDIS. When John Barrowman returned to the proceedings as Captain Jack for the series finale, things went from bad to worse. How on Earth did they get through an entire scene without corpsing? Professionalism at it's best.