It's a shame we see so little of the actual Tyrant of France in "The Tyrant of France", because the scenes with Robespierre are the best part of this episode. Despite the general storybook atmosphere of the story, Spooner isn't interested in reducing Robespierre to a one-dimensional caricature; instead, he makes good use of the old truism that every villain thinks he's a hero. Spooner doesn't shy away from depicting Robespierre as a paranoid mass murderer, but he also shows the guilt lying not far beneath the surface of Robespierre's brutality. He knows that he's murdering people by the hundreds, he knows that history will remember the bloodshed, but he sees no alternatives at this point. Better to be remembered as a monster who founded the New France than a monster who failed, right?
Hartnell's Doctor is great in this scene, and he's also great in trying to avoid a reprise of it. His bluster is charming and hilarious, even as we catch wind of the fear that underlies it. The Doctor knows full well that he's stepped in it much deeper than he planned to when it was originally a hastily-improvised disguise to intimidate the jailer into releasing a few prisoners, and his attempts to bluff his way out are tense and funny at the same time. Even so...
Look, it's not that the episode is bad. It does an effective job of moving all of the pieces around in an interesting fashion, what with Susan getting sick (although again, this story does seem to mark the point where they stopped even trying to do anything with the character other than keep her around as a convenient warm body to be dragged from one peril to the next) and her and Barbara getting recaptured and Ian pursuing his spy plot and the Doctor trying to avoid getting exposed as an imposter...but it's pretty clear by now that moving the pieces around is all that's happening. There's no real story here; Stirling's plot is so slight that it's barely taken up a few minutes of each episode, and there's no real sense of urgency or momentum to it. Ian is looking for Stirling to give him a message because some guy he met asked him to. There's no emotional depth to it, and everything else in the story is just a run-around until the allotted number of episodes is up and everyone can escape to the TARDIS.
Again, this is primarily a function of Spooner having jettisoned the series' previous approach to the historical without yet knowing what to replace it with. 'The Romans' will make better use of the farce structure that is essentially being developed here, and 'The Time Meddler' will inaugurate what generations of fans will come to know as the established structure of the series, "aliens meddling in important historical events". But here, he's got six episodes to fill, and very little to fill it with. He's already getting better at learning how to hide the padding, but it's unfortunate that there's still so much padding to hide.