There is a shocking variance in tone between the two portions of the scene where the Shadow interrogates The Doctor. The initial part of the scene, where The Doctor is trapped in a cell and zapped with electricity when he gives the Shadow the wrong answer, is intense and exciting. We can see that The Doctor is in distress, so we, the viewers, feel distress. It's one of the rare occasions in the later Tom Baker era where it appears that The Doctor is in genuine danger.
But once the Shadow lets The Doctor out of the cell to open the TARDIS and fetch the five segments of the Key to Time, the drama completely dries up in a sea of Tom Baker's usual bluffs and off-key remarks. Funny, yes. Entertaining, yes. But the Shadow has all of two minutes to appear as an adversary worth worrying about, and any threat that we thought he might have is now gone.
Yet more characters - The Doctor, Romana, and Shapp - try and get Merak to keep prattling on about Astra in this episode. "Us-trah!" seems to be the only thing Merak says from Episode Two onwards. Words to live by, Merak. As Shapp says to you, "Merak, if I were you, I'd try to exercise a little more self control".