Complex sentence: Peter Jackson's film' (2002) deviates somewhat from Tolkien's story by portraying Théoden (played by Bernard Hill) as being actually possessed and magically aged by Saruman (Christopher Lee).
Split into simpler sentences: Peter Jackson's film adaption of deviates somewhat from Tolkien's portrayal of Théoden, who was played by Bernard Hill. However, unlike in the books, the Lord of the Mark is actually possessed and prematurely aged by Saruman (Christopher Lee).