In theory, certainly, but in practice... With the pillow and the piano, you're the one casting the spell, not the pillow. *smiles* Much harder to concentrate on casting the spell when you're being re-shaped at the same time. And... It's not simply a matter of whether you're good at Transfiguration or not. It's... *sighs, starts again* Your personality. You need to be the sort of person who can let the change in. Who can be open to the change, and welcome it in, and... *tries to think of a way to say this without mentioning possession-by-Voldemort directly*
For example, a werewolf can't become Animagus, because something's already forced a change in that person, and it will fight off any subsequent change. So.
*briskly, in a 'nothing-to-see-here' tone* First, you have to be able to cast the spell while you're being transformed yourself. Second, you have to want and be open to the change. And finally, *said with no more emphasis than the first two* if there's something there already -- other than just you, that is -- then the change may not come at all.
*vaguely disturbed by this revelation -- not the news that Snape and Q have talked about him (which isn't news), but the fact (and associated implications) that he shares the same form as this other Sirius that nobody seems to want to talk about -- but covers by drinking his coffee* I didn't consciously choose the form. It's possible to 'see' what your form is likely to be before you commit yourself to the prep. work, but I -- We chose not to look. *small smile* In case it put any of us off. *wonders if Peter would have re-thought the transformation if he'd known his form in advance, and what (if anything) would have been different*
And... yes, it is vague. Deliberately, no doubt. *grins* Almost as though they want to make sure you learn from an instructor, rather than out of a book.
What form does your Patronus take?
Those options aren't mutually exclusive ;) *huggles Sirius*