So this is the one where Barbara the history teacher just straight up threatens to slit a man's throat within the first thirty seconds.
The thing is, Jacqueline Hill plays the scene absolutely straight. There's not a hint in her voice or her expression that she's bluffing, or that she's frightened or nervous about the act. This is really a watershed moment for Barbara, a point where she realizes that if she's going to play a game this dangerous then she has to be willing to be dangerous herself. When you watch Barbara in that scene, there is absolutely no doubt in your mind that she will leave Tlotoxl bleeding out on the flagstones if she has to. It's no wonder the man backs down.
That's just the opening scene, too. This is an episode that really specializes is racheting up the tension on every level. Each scene raises the stakes in its own way. Ixta makes it clear that he has every intention of murdering Ian in cold blood (while, in a series of events that are both bizarrely chilling and hilarious, insisting that this shouldn't affect their friendship). Tlotoxl moves directly to his next plan, convincing Tonilla to help him murder Barbara with poison. Barbara in turn foils that gambit, and plays her own counter--she tells Tlotoxl directly that she's not a goddess, and dares him to denounce her openly. Each move and counter-move draws Barbara more deeply into events, forcing her to put more and more at risk.
Then of course there's the Doctor. Much has been made of his scenes with Cameca--how much did he understand of the cocoa bean ceremony? How much of his feeling for her is genuine, and how much is him playing her along to learn the secrets Ixta's father may have told her? To his credit, Hartnell doesn't play it in a way that makes any reading obvious. He understands that there's value in suggesting both sides; it prevents the character from seeming unsympathetic or foolish. (Although the near-spit take he does while drinking his marriage proposal is great.)
It takes Ian, in the end, to confront Barbara with the truth she's been avoiding, the blunt fact that human sacrifice is a way of life for the Aztecs and not just a superstition. It's not just because he's human and the Doctor is a pompous and arrogant Time Lord (yes, yes, I know, they're not called that yet). It's because he's been out there among the people. He's seen who they are, the bad as well as the good, and understands that Barbara's view of them as clay to be molded into "civilized" people is patronizing and false. His own speech about Autloc being "the civilized one" isn't without its own prejudice, but at least he understands that they're not children who need the guidance of a Great White Mother; they have a way of life, and they don't plan on changing it any other way except through force. When he breaks through Barbara's conviction, she finally understands just how much danger they're in and how entangled in events they really are.
But Susan doesn't. She still sees herself as fundamentally apart from the Aztecs, a visitor and not a part of their society. She thinks she can take and leave their customs, learn their religion and not have to follow it. She still thinks of herself as...well, maybe not a Time Lord specifically, but at the very least an alien. It's no wonder Tlotoxl chose her to spring his next trap. Compared to the choice presented to Barbara at the end--will she see her deception through all the way to allowing Susan to be maimed and tortured?--Ian's "death by flooding" predicament that forms this week's cliffhanger seems relatively minor. Presenting both back-to-back, though, leaves the story wound up to breaking point.