"Closer to sleep than meditation. The formula," and Remus emphasises this heavily, because if Q is taking anything, it should be the preparation entire and not just the catcus juice (although that can be fun, too), "simply lowers your guard if you're the sort of person who normally has trouble doing that."
"And you don't visualise yourself in another form," Sirius cuts in impatiently, because he's explained this already. "You experience the dream as though you're already in the other form, which means you don't actually see yourself."
"Well, you might, if you come across a mirror or some other reflective surface in the dream," Remus says, with a placid smile.
Sirius ignores Remus' sarcasm (and it is sarcasm, the bastard, even if it's said Nicely) because he can't believe Q is actually asking him about the dangers of not being able to change back, as though it's only just occurred to him# -- after all, this is exactly why Sirius suggested he started with the one-animal-into-another task. And now that he thinks about it, Q hasn't actually said whether he's made any progress with that or not.
Remus can see Sirius winding up to say something unnecessary, and starts talking before Sirius can even open his mouth. "It's one of two risks. The first is physical -- where you may transform incompletely, and that can be quite... unpleasant. I'm sure you've seen the pictures -- every text on that transformation is usually careful to include several illustrations of what can go wrong. And the second is mental. As difficult as it is to transfigure something that's sentient into something else that's sentient is holding the consciousness fixed while transfiguring everything else around it.## It's not commonly used magic, and it's not taught even at the N.E.W.T. level, so a lot of puative Animagi have trouble with it."
Sort of wearily, Sirius interrupts again, and says, "It's not a safeguard. It doesn't protect you or ensure that you keep your own mind -- it only increases the likelihood. If you normally find it difficult, then taking the formula will mean you don't find it difficult. If you normally find it extremely difficult, then taking the formula will mean you 'only' find it difficult. And we won't know whether you even need it or not until you try."
"With someone nearby," Remus adds quickly, because whatever Sirius may sound like now, he will supervise and help with the return transformation if needed, "in case you run into difficulty."
Neither Sirius nor Remus are certain whether Severus' reference to a non-invasive diagnostic is one they already know, or something else, so they leave Q's question to Severus to respond to.
# This is not to say that it has just occurred to Q -- only that that's how it sounds to Sirius :)
## I think JKR has actually said that when fake!Moody transfigured Draco into a ferret, Draco!ferret didn't remember it was Draco!human, and acted entirely as a ferret -- in so far as it had the opportunity to do anything other than be bounced around :D