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).
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Due to the vague question (and due to Jackson's film messing with the plot) this question may mean any of the following: Peter McKenzie (Elendil), Harry Sinclair (Isildur), and Viggo Mortensen (Aragorn).
As a side note, Denethor, the Steward of Gondor was played by John Noble.
Viggo Mortensen played Aragorn, who was crowned king of Gondor at the end of Peter Jackson's The Lord of the Rings: Return of the King.
John Noble played the Steward of Gondor, Denethor.
The Lord of the Rings The Return of the King was released to theaters in 2003. The movie starred Orlando Bloom, Sean Astin, Ian McKellen, and Viggo Mortensen. This was the third movie made for the trilogy.
Aragorn Elessar is the king of Gondor and Arnor returned. Theoden, and later Eomer, is the king of Rohan, Gondor's neighbor and ally.
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.