system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] How did Saruman possess Theoden?
gpt
In the books - he didn't. The part with Saruman speaking through Theoden's mouth doesn't  happen. 
Theoden has been lied to and demoralised by Wormtongue. Gandalf cures this by showing Theoden the truth. He shows him the light  through  window. He gets him to stand and walk. He gives him his sword to hold. He shows him the landscape - its not that dark.