Wuuuuut.
It had my understanding up till spells became sentient and your understanding of a spell involved if you had it prepared. Bull on that. But a sorcerer, for example, who knew the Fly spell should by rights know it inside, outside, back to front and all, and a wizard would be able to have close to the same knowledge, if he has his spellbook. Otherwise, the spells he reviewed that morning and pre-casted (my personal explaination for why spell casting works as it does, when you prep spells you pre-cast them and you actually casting them is filling in the handful of blanks needed (center of fireball, for example) and such) are better focused in his mind, of course.