A Probably too Sympathetic View of Gríma

[Keeper of Dol Guldur]

Saruman did not manipulate him, but rather promised him rewards. And Theoden was never under Saruman's "control", not in the books. There is no exorcism in the books, but rather wise words by Gandalf.

Ah, but do you remember the wave of tiredness, the dark force of will that Aragorn, Legolas and Gimli felt when they entered Rohan and began moving toward Isengard?

Saruman was capable of exerting his presence and weakening their wills ... and speeding along his orcs, probably in the same way that Gandalf did the complete opposite, strengthening the wills of men, and intimidating orcs with no more than sheer will (you saw this more when he became "White", naturally).

So, if Saruman were to bend that focus onto one man, Theoden (who he could presumably monitor using his Palantir, which is what the movie logically came up with as the tool of the "sorcery" involved) then he may very well have been applying just a bit of pressure on Theoden.

I don't think he could literally possess the mind of any man ... but weakening wills seemed to be right up his alley, and with Wormtongue giving logical explanations and manipulating events so that Theoden simply thought he was growing older and wearier ... it seems like this 'spell' (it wasn't really a spell) could have worked.

Saruman's 'menace' was a viable part of Theoden's weakness. Between that shadow of fear, and force of will, Wormtongue's meddling and bad times, Theoden was subdued.

Anyway, Grima son of Galmod was Rohirric, he was devious, manipulative and crafty from the beginning ... and Saruman knew exactly how to play him.

It wouldn't take much to see that Wormtongue had a fancy for Eowyn ... Saruman could have used that. Also, as a scrawny, weaker man growing up among tough, militaristic, athletic lads like Theodred and Eomer, Wormtongue probably was a bit of an outcast in youth, that would explain why he felt like he should turn on his own people ... it's likely Theodred and Eomer weren't especially kind to him, even in youth ... and he was getting a little payback.