As there are various kings in the LotR (answers refer to Peter Jackson's live action film adaptation):
-(appearing in the prologue:) Peter McKenzie (Elendil), Harry Sinclair (Isildur), Mark Ferguson (Gil-galad, High King of the Noldor [=a division of elves])
-Bernard Hill (Théoden, King of Rohan)
-Viggo Mortensen (Aragorn, later King of the Reunited Kingdom of Arnor and Gondor).
Bernard Hill .
Theoden is the King of Rohan for a period of time before he is killed and his sister-son Eomer becomes King aftersaid.
The king of Rohan, theoden.
Ok I am guessing you are thinking of King Theoden of Rohan, when Saruman is like posessing him. Hope that helps
The Lord of the Rings: The Return of the King.