None taken, Sirius sends back to Severus, while passing the baton to Remus.
"There's likely some confusion between the various sources, since there are two formulas that are useful to the transformation," Remus says, knowing that Sirius will not have made any of this clear. "The one that uses the cactus juice is to, as Sirius said, induce the animal spirit. I don't think you'll find much about that in a transfiguration source -- you'd have better luck trying the Divination stacks at F&B. Even there, as you rightly say, you don't have to take it, but it makes things easier."
"Especially if yours is the sort of mind that would find it challenging to allow something else that close, let alone invite it in," Sirius adds, since this was the reason he'd suggested it to Q in the first place.
"Yes, easier but not necessary. The same goes for the second formula, which is for the transformation itself. It increases the likelihood of retaining your human consciousness when you shift forms. Some people who genuinely struggle with that part of the transformation -- and it is critical, really, since you can't change back unless you've retained your own consciousness -- have the formula thickened and inked into their skin, like a tattoo, so that it's always there to assist them."
Remus probably doesn't sound any clearer than Sirius here -- will try to differentiate him more (he is much more of a teacher than Sirius is, but he still has me typing for him) :D
Anyway, the intent is that he makes more sense than Sirius, even if that's not readily apparent :\