John Arnatt, the second of four actors to play the part of the Chancellor, is the definitive Borusa, and the one most fondly remembered by fans. It's not surprising, however, as the Borusa of this story is much more loyal to The Doctor than the Borusa of The Deadly Assassin, much more effective than the Borusa in the later Arc of Infinity, and much less evil than the Borusa of The Five Doctors.
It's almost touching to see the lengths Borusa goes to to protect The Doctor by secretly lining The Doctor's quarters with titanium, and pulling a gun on The Doctor's friends because he isn't 100% convinced that they are not there to kill The Doctor. He even tries to protect The Doctor from himself by doing his best to keep the Great Key of Rassilon hidden. Arnatt and Tom Baker also have a good rapport onscreen, helping to further my theory that Baker seemed to get along much better with older people (an observation that proved correct offscreen).
The Sontarans, for the most part, are quite impressive in this episode because they are treated with a great deal of respect by the characters, the story, and the production crew. Gerald Blake chooses to shoot against their short stature, often using camera angles from below. The actors playing the Sontarans also give it their all. They also have two advantages over the previous Sontaran appearances in that there are more than one Sontaran, and while Kevin Lindsay is still the definitive Sontaran, his heart condition meant that the Sontarans were nowhere near as active as they are here. And to top it all off, Dudley Simpson comes up with another good theme for them, too! This episode is probably the strongest portrayal of the Sontarans, or any other alien race, seen on the series for some time.