He had been watching Alastor Moody since the first day of term.
Not obviously. Not in the way that invited observation of the watching. He had learned in two years of preparation that the most important quality in sustained assessment was invisibility — the ability to look without appearing to look, to note without appearing to note, to accumulate data points in the specific patient way of someone who understood that a single observation was almost never sufficient and that the picture required time.
He had spent August working through the dueling memories until the real Moody's patterns were as familiar as his own. The economy of movement — a Senior Auror who had survived thirty years in active field work moved with the specific deliberate efficiency of someone who had learned, the hard way, that unnecessary motion was a liability. Every step placed. Every spell delivered from exactly the position that maximized the next available option. The eye — the real magical eye, not the performance of it — moved in continuous assessment, never resting, but in a specific rhythm that had become unconscious from years of use. The paranoia was real and had structure: Moody checked his flask because he had actually been poisoned three times and had decided the cost of the habit was lower than the cost of the alternative.
The man who arrived at Hogwarts on the first of September moved like Moody and talked like Moody and had Moody's eye in Moody's socket.
He did not check the flask the same way.
It was small — the kind of thing that was invisible if you weren't looking for it. The real Moody checked the flask with the left hand, always, because his right arm had been damaged in the Maddock case in 1979 and had never fully recovered the fine motor precision the left hand. The man in the DADA classroom checked with the right. Not consistently — not every time, which would have been its own signal. Inconsistently. As though the habit was learned rather than ingrained, running correctly most of the time and defaulting to the other hand when attention was elsewhere.
Ron noted it on the third day and said nothing.
He watched through September with the patience of someone who had more data points than he needed to be certain and was collecting further evidence anyway, because certainty without documentation was only certainty to himself. He needed specifics. He needed the gaps to accumulate into a pattern that was actionable when the moment arrived.
In DADA, Crouch Junior — and it was Crouch Junior, Ron was certain of this by the second week, the flask tell confirmed by six further observations of right-hand use under pressure — was a genuinely excellent teacher. This was not surprising. Bartemius Crouch Junior had been, by all available evidence, extraordinarily capable before his arrest. Whatever Voldemort had done to him with the Imperius had not touched the ability. It had touched everything else.
Ron sat in the front row and paid attention and took notes and gave nothing away.
He was good at this now. Two years of Occlumency and the specific practice of being in a room with people who were watching him and allowing them only what he intended them to see. Crouch Junior looked at him the way a DADA teacher looked at a strong student — with assessment and a degree of interest. Not with recognition. Not with suspicion.
Good.
He needed it to stay that way until he knew exactly where the gap was.