He was very different in the book. Book Theoden actually rode out to confront Saruman, not to hide his people at Helm's Deep. Afterwards, he fully intends to gather his forces and head to Gondor. He actually sends a rider with the red arrow (a sign that Rohan is on its way), but that rider is killed and Denethor does not receive word of their approach. Ghan-buri-Ghan takes them on a secret path through the hills and brings them to the battle at Minas Tirith. At no time in the book does Theoden consider anything but death in battle. He never hesitates or reconsiders--from the moment that Gandalf revives him at Edoras, he sets out to defeat the enemy or die trying. Movie Theoden is constantly indecisive and second guesses himself. "Who am I, Gamling?" "Why should we help Gondor?" "I was not the one who led my people to victory at Helm's Deep, it was Aragorn and I'm just a failure, boo hoo." They rewrote his character and made him struggle with doubt and insecurity so that he could "earn" his death--"I go to my fathers. And even in their mighty company I shall not now be ashamed." Like movie Faramir, he had to have some internal struggle to overcome before being allowed to be a purely strong character. PJ just likes people to be flawed and have some character development alien to the book. It supposedly makes the character more layered in the movie, even if it means a radical departure from the character that Tolkien envisioned and created. In Theoden's case, the character is made much weaker by PJ's changes, yet at the same time more sympathetic. We can relate to him more, and therefore feel more compassion when he dies. Also, there is more suspense created as we wonder what he will (or won't) do. I understand why they changed his character for the sake of the movie, I just don't necessarily prefer movie Theoden to book Theoden.
