Théoden (Bernard Hill)
Talk about a transformation. When we first see Théoden he's old and wrinkled and generally haggard looking. His hair is hoary and frazzled and his eyes are milky and clouded with old age. Mentally, he's non-existent. Wormtongue whispers in his ear and he just sits there, soaking in the poison. When Erwin tells him of his son, Théodred, being badly wounded, he can't even master a shrug.
The single time he does show a bit of life is his defiance of Gandalf. Calling him "Stormcrow," he laughs when the wizard attempts to throw off Saruman's hold of him. This laugh is almost demonic; he bares his crooked teeth and throws his head back. It's clear Théoden is not himself; this is Saruman we're dealing with.
So, where is the true Théoden? Well, when we see him change in front of our eyes. His hair grows brown and thick, his skin tightens, and his eyes now glow with life. Gandalf, being almost as manipulative as Saruman, suggests, "Your fingers would remember their old strength better if they grasped your sword." And when he does, he goes after his deceiver, halted by Aragorn just before he cuts him down.
But despite this display of aggression, Théoden doesn't want to lead his people out to open war. With Éomer's riders gone, Théoden believes he has no choice but to risk traveling to Helm's Deep, which the Rohirrim have defended for generations. The journey from his resurrection to the retreat of the Uruk-hai is a difficult one. Théoden is at first steadfast in his power and position as king, telling Aragorn, "When last I looked, Théoden, not Aragorn, was King of Rohan." (Translation: "Sit down, son!")
But as he hides in the keep of Helm's Deep, with orcs ready to bust through the doors and his army all but defeated, Théoden questions himself: "Who am I, Gamling? You are our king, sire. And do you trust your king?" As when Théoden wonders, "How did it come to this?" it seems like he's lost all hope in himself and in his people.
But Aragorn doesn't lose faith. He convinces Théoden to go out in a final ride of glory, and the King's valor is renewed by the appearance of Éomer and his Rohirrim. He yells, "Fell deeds, awake. Now for wrath, now for ruin and a red dawn."
He's still a warrior king, who will defend his people at all costs.