[@Rune_Alchemist] It really depends a lot on the mage, actually. An unexperienced mage may actually be possessed [i]possessed[/i] by the spirit, and they will experience symptoms of either a complete change of personality if their will is not strong enough, and if they fight it back, then they will experience something akin to what we see in exorcist movies or such. Convulsions, puking, etcetera, etcetera, in a fight for dominance against the spirit. 

An experienced mage has the spirit either firmly on a leash or has a contract with them that allows the two of them to live in a same body. 

Spirits possessing others usually use a lot of stamina and mana to manifest and use so it is often used as a trump card.