Unprepared and Daily spells, and Rituals (4th edition): In the 4th edition, wizards only needed to prepare their most powerful attack spells, those which could be used only once a day, and their utility spells.
Casting: When the need calls for a certain spell to be cast, wizards will allow their thoughts to retreat back into their consciousness in order to obtain it, and it often appears that wizards are in trances while they are casting.