[The Saucepan Man]

I seem to recall reading somewhere that Grima Wormtongue was a servant in Theoden's house who rose to be his trusted adviser though deception and false-speaking. I believe that he was tempted into league with Saruman by promises of power over his countrymen and also that Eowyn would be "given" to him. Presumably he must have visited Orthanc at some time for Saruman to have recruited him - this rings a bell with me, but I cannot find the reference.

Eleanor is of course correct - Saruman never "possessed" Theoden in the book. Wormtongue, his servant, did just what his name implies. He wormed his way into the King's trust with his false words and deceived him into inaction and hopelessness.