A megalomaniac believes themselves to be the single most important person in the universe. Nothing is more important than the megalomaniac and everything around them must be done according to their whim. Failure to comply with the dictates of a megalomaniac can often result in rages or actual physical assaults by the subject.
When the player is under the effects of another personality, they should be treated as an NPC. In some rare cases the player and the gamemaster can work out the second personality and allow the player to roleplay this. This does not however constitute an entire new character that can be “turned on” at will.