One of the all time great "Holmesian double acts" appear in this story (with each story Robert Holmes writes, that title becomes more and more contested), as Garron and Unstoffe are two wonderful characters who complement each other superbly, and are very well played by Iain Cuthbertson and Nigel Plaskitt.
The scene in the relic room is an absolute hoot, with Plaskitt's shrieve from Somerset providing most of the laughs. What I love most about the scene, though, is how surreptitiously The Doctor and Romana sneak into the room behind the Graff Vynda-K and watch the proceedings unnoticed. (Look, too, how Tom Baker presses his nose against the glass of the relic case).
One bit of acting that does let the side down, though, is Tom Baker's reaction shots right before the credits roll, after The Doctor's execution has just been ordered by the Graff. The look on Baker's face isn't one of terror and panic, but one of sheer comic disbelief. You never once feel afraid that The Doctor is going to actually be shot in the next episode. And, of course, he won't be shot, but we should at least feel some peril for The Doctor, as he should feel some peril for himself in that situation. Baker's increasingly humourous approach towards the part of The Doctor has resulted in some uproarious moments, but it has also decreased the amount of real drama present in the show, and this moment is an example of that.