Last night's viewing - Line of Duty, BBC2; Twenty Twelve, BBC2

It's all very well slitting the throat of one of your major characters at the end of episode two, but it does present you with something of a problem in the succeeding episodes, as Line of Duty has been demonstrating. There are upsides to such narrative ruthlessness, obviously. If Gina McKee can cop it, the audience will be thinking, then frankly nobody can be assumed to be safe.

But that vivifying uncertainty brings with it an increased appetite for the unexpected to happen. Having been fed a deliciously savage cliffhanger once, we find that we rather fancy another one (they're so moreish, those things). And if we don't get one (and we shouldn't) we might start to get a bit picky.

I've been fairly preoccupied myself with The Case of the Disappearing Contusion. After all, DCI Gates was left semi-conscious next to the body of his dead lover after being whacked round the head with a baseball bat. And yet the following day he was displaying no signs of damage at all, despite the fact that he has the sort of close-cropped skull that tends to reveal the forceful application of blunt objects. This was very helpful for DCI Gates, who has been pretending that he turned up at the scene to find only an enigmatic blood stain. But it's not at all helpful to those of us striving to think of Jed Mercurio's drama as an exercise in procedural realism.

There are more fruitful enigmas, though, exemplified by a nice scene last night when Vicky McClure's undercover spy Kate tried to persuade Gates that he should unburden himself. For a moment, he paused, on the brink of what looked like a confession, and then stopped: "No... sorry, Kate," he said. "Paranoid." It was impossible to tell whether the word was a description of him or her, whether he was effectively denying her charges or acknowledging that he was now lost himself, arrived at a place where he doesn't know who to trust to keep his untrustworthiness a secret.

Lennie James's performance helps a lot, consolidating the script's portrait of a man undone by his own swaggering confidence. It has shreds of desperation in it that persuade you that this might be a good man who's just cut one too many corners, but also an angry arrogance that lets you see why he felt able to ignore the rules. His fury at the implication of his children in the investigation is both a badge of character and a potential motivation for crime. And here the drama wobbles unsteadily in ways that are entirely to the good. We can be fairly confident that this is a study of hubris, of the corrupting effect of your subordinates' unquestioning admiration. But Gates could be guiltier than we've assumed without breaking the terms of our contract. Added to which – having taken a week off torturing his central characters – Mercurio again demonstrated that nobody is safe with a very nasty scene indeed.

Twenty Twelve also exploited the potency of deferred pleasure last night with the return of Sally, Ian's haplessly lovelorn PA, recruited by the perky Daniel after he was headhunted for Lord Coe's team. Her arrival in Ian's hospital room, in the midst of a cloud of self-deprecation and apology, was wonderfully touching. This was partly down to Olivia Colman, who can do more by lowering her gaze than many actors can do with their entire body. But it was also to the credit of John Morton's script, which sits very sharp satire on a foundation of beautifully understated character studies. Without the latter the former might get a bit thin. But with them it is irresistible. He can write a punchline too, one of which might serve as a useful slogan for the Home Office team currently dealing with security: "If we get this wrong we're in danger of running out of feet to shoot ourselves in."