One of the central themes in the Lord of the Rings trilogy is the question of fate: whether or not the characters who lead them down certain paths were always correct. Frodo chooses, right at the last moment, not to throw the ring into the fires of Mount Doom to destroy it, a decision that jeopardizes the fate of all of Middle Earth. However, Gollum chooses to pursue Frodo and Sam until the very last moment, and it is actually Gollum who falls in the
It is possible to speculate that these were both conscious decisions that led to certain outcomes, but it is also possible that fate guided them to this point, and fate spared Frodos' life and took Gollums instead. Gandalf does say much earlier in the trilogy something tells me that he still has some part to play in all of this which sounds like there is already a pre-determined path ahead of each of them.
Many of the characters are aware of this burden and seem to carry it heavily. Galadriel for example sees visions in her magic mirror, which helps her pass the exam when presented with the ring of power right in front of her. Denethor, on the other hand, sees visions of the future in a Palantir, which leads him instead to hopelessness and futility, because he believes that the disastrous consequences of Sauron's becoming ruler over the world are inevitable.
Bilbo and Frodo are, of course, the most obvious choices, for protagonists who appear on one trajectory of a peaceful and quiet life in The Shire, but who are thrust into completely different adventures that they never expected. Bilbo almost misses out on the adventure because he changed his mind.
Eomer, a famous rider in the Lord of the Rings, goes through several difficulties that he would have never imagined. He is imprisoned by Wormtongue due to the ill-gotten uncle's decree, he must flee his sister when he discovers her in the clutches of the lecherous traitor, and he takes over a kingdom that he had never intended.
Eomer never should have been crowned, because, unlike Boromir, who was hounded and trained and transformed into a ruler, a noble and brave man capable of governing the kingdom of Gondor, he never had such training and emphasis placed on him. This duty was always meant to fall to his cousin Theodred, the king's son.
Theodred is badly wounded in a battle against the orcs, and dies of the poison in his veins (though whether or not the poison is from the weapon in question, or part of a malicious plan by Wormtongue has long been debated) The throne therefore passes to Eomer, who is significantly under-equipped to deal with it, because she has all of the same battle skills and life experiences as him, yet has a higher capacity to make friends among the surrounding kingdoms,
Eomer does a wonderful job, and leads Rohan to prosperity by fostering the Reunited Kingdom in the Fourth Age of Middle Earth, considering that fate imposed him, or Eru, the creator of all things from The Silmarillion, or destiny, as the power that guided them throughout this journey. So perhaps fate, or Eru, the creator of all things from The Silmarillion, or destiny, already had this in store for Eomer from the start, because he was always capable of greatness.