Doctor Who: 13 Cool Details In David Tennant & John Barrowman's Last Of The Time Lords Commentary
5. The Hilltop Scene Was Added At The Last Minute
After the Doctor is de-aged at the end of the episode, the Master activates Jack's vortex manipulator and teleports to a rocky hilltop, bringing the Doctor along with him.
This leads to a nice little moment that gives the two characters a final one-on-one standoff, and it fits very smoothly into the flow of the action.
But originally... it was never meant to be here at all.
While discussing this scene, Tennant mentions that it didn't exist initially, and that it was only added "at the last minute".
Why? Well, it just wouldn't be an episode of Doctor Who without multiple scenes set in a quarry (or a place that looks like a quarry) would it?