How should Liches be characterized? Comically evil all the time? Villainous but with many strange idiosyncrasies which can make them seem less dire in some cases? Or should Liches always represent an extreme threat to the player whenever encountered?

In terms of magic-users you may find operating in the material plane, are Liches the peak? Can they go a little senile over time? Like, mistaking your party for lab assistants who died centuries ago?