Doctor Who: 13 Cool Details In David Tennant & John Barrowman's Last Of The Time Lords Commentary
2. This Mysterious Hand Was Played By The Production Manager
Whenever you see a shot of a character's hand, foot, or leg (basically, any body part that looks quite nondescript) there's a good chance that this scene was filmed with a random extra or a random member of the crew - just anyone who happened to be around on the day.
There's no point calling in the lead actor just to film a two-second shot of their hand, after all. It's often quicker and easier to just use someone else.
Doctor Who has utilised this trick a few times - for example, in The Eleventh Hour, Steven Moffat once pointed out that the Doctor's hands weren't played by Matt Smith - and it happened again in Last Of The Time Lords.
The hand that picks up the Master's ring at the end of the episode wasn't played by an actress - it's just production manager Tracie Simpson, with her fingers decked out in a few layers of red nail polish. Very sinister indeed.