Hello,
Do you think a ReMe (base 15 or 25 to control another's mind) can let you cast the possessed magus' spell? Yes? No? and what would be your explanations?
IMS the SG states a big ReMe spell (base 25 to control human mind) can't force the possessed magus to cast a spell, because of the soul limit. I agree with him because i wasn't sure and that's only for a NPC big magus.
But i'm quite curious about it, and, although i read the whole forum these last days, i never found a topic about it ^^
I would say a strong enough spell could force a magus to cast a spell. The key here is making the mind want to cast the spell, perhaps even affecting only the magus' feelings (into loyalty and so on).
But I'm not your storyguide. Whatever your storyguide says goes. Not allowing casting a spell is a perfectly fine interpretation, giving a place for the soul's Free Choice separate from that of the mind. In this case, however, I'd suggest also limiting the use of other characters' skills or, at the least, supernatural powers.
Bottom line, what Y7R said - SG decides (but the Troupe can speak up too!)
However, there are some spell effects and other comments in the rules that imply that it could be tough. Look at ReMe Exchange of the Two Minds. It is strongly implied that the caster must "concentrate" while casting a spell - I'm not convinced that someone controlling the mage's actions would know what to concentrate on, etc etc, to the level of manipulating magic themselves via that mage.
So, I don't think that a ReMe could give the caster perfect control over a mage, so perfect and complete that the caster could work the mage's spells like a remote puppet. A much better approach, imo, is to somehow manipulate the mage's emotions, beliefs, memories, etc so that the mage wants to, needs to cast that spell himself. (This might also give a Personality roll if the spell were completely contrary to the mage's natural desires.)
The end result is the same, but the difference in how the effect is achieved is, imo, important. It's the difference between treating magic as a "just another action/skill/knowledge" vs something more.