Doctor Who: 13 Cool Details In David Tennant & John Barrowman's Last Of The Time Lords Commentary
8. During The Flashback Shots, Tennant Whispered The Plot Of The Episode To Agyeman
There are several "flashback shots" dotted throughout the episode, which cut back to the final scenes in The Sound Of Drums.
Here, we see the Doctor whisper something in Martha's ear... but we don't actually hear it for ourselves.
In the end, we learn that he says "use the countdown", but in reality, Tennant didn't say this at all. Instead, he whispered the entire plot of Last Of The Time Lords into Agyeman's ear, purely so the director - who left the camera running while Tennant spoke - had a lot of footage from which to choose his flashback shots.