Many abilities and spells can cloud the minds of characters and monsters, leaving them unable to tell friend from foe -- or worse yet, deceiving them into thinking that their former friends are now their worst enemies. Two general types of enchantments affect characters and creatures: charms and compulsions.
Compulsion is a different matter than charm. A compulsion overrides the subject's free will in some way or simply changes the way the subject's mind works. A charm makes the subject a friend of the caster; a compulsion makes the subject obey the caster.
Regardless of whether a character is charmed or compelled, he won't volunteer information or tactics that his master doesn't ask for. If a 1st-level wizard happens to have a staff of fire, the vampire that is compelling him doesn't know that the staff is there and can't tell the wizard to give him the staff or use the staff on his former friends. The vampire, however, can say, "Hand over your most powerful magic item."