I'm going to followup on Lance's post on Lucas and Tolkien from the other day, but move beyond naming. As I recall, there's a late appendix in ROTK about Translation that is really a caution that the names - place and personal - we're reading are anglicisms; Bilbo was originally Bilba, for example, and changed because the "-a" ending is, for us, generally feminine. Tell that to a Lithuanian.
But Lance really does zero in on the most compelling characters in LOTR (Gollum apart). The ruling houses of Gondor and Rohan - the Steward Denethor, his sons Boromir and Faramir, and King Theoden and his nephew and niece Eomer and Eowyn - bear the human brunt of Sauron's malice, but it's been poured into their ears in a trickle, and each responds differently.
Saruman is a different case, as he's not really human. Like Gandalf (and, indeed, Sauron himself), Saruman is of an order of immortal servants to the (to use Buffy terms) Powers That Be - like but lesser. Sauron turned Bad early on, and spent his career accumulating power for domination; the skills he employed to embody that power in the rings he made are the same that Saruman, before he was sent over the Sea, studied himself. It wasn't, as Lance originally assumed in his post, that Saruman didn't need the Great Ring - he made quite enough mischief without it, and in the Sauron style - but he knew more about it than the other wizards, or the High Elves, e.g., Elrond and Galadriel, who sat with him in Council, and who deferred to his authority while he lusted and schemed.
And then there are the Palantirs. It's really too bad that it's only in the Extended Ver of the ROTK movie that you get a glimmer of the importance of the seeing stones of Numenor. Glorified crystal balls, there were originally seven, and at the time of the LOTR, Sauron has at least two (including the Master Stone that could see all the others), Saruman has one, and Denethor has one, which he has kept secret. They are controlled by the strength of will of the user, and if you are the more powerful, you can mind-fuck your correspondent. That's what happened to Saruman - he became a conduit for Sauron's malice, feeding his own hubris, and this flowed into the court of Rohan via Wormtongue. Until Gandalf intervened and helped Theoden cast it off, the poison had already threatened Eomer in his resistance to it, and thrown Eowyn into despair. Even after Theoden revives, it drives Eowyn to nearly suicidal heroism.
Something similar happens in Gondor, but with echoes of that kingdom's own Numenorean roots. Sauron, whose temptations of the Numenorean kings destroyed that analogue of Atlantis, goes to work on Denethor. Shows (selective) of overwhelming strength, hints of usurpation by some false King in Exile (Denethor is only a Steward, recall, at the foot of an empty throne), suspicions of rival advisors (i.e., Gandalf) - these undermine Denethor's noble entitlement and feed his expectations of doom. They also poison his relationship to his sons, driving him to favor Boromir for his martial strength and to devalue Faramir for his better judgment. The LOTR movies are especially good on Boromir (whose fall, I think, dominates the FOTR installment - yay for Sean Bean!), and, in the ROTK Extended, shows the brothers together (with uncanny resemblances). Without knowing just how much Denethor has been worn down, however, he seems more petulant and arbitrary a father and ruler than he was in the book, and his suicide hasn't quite the pathos of the original. Great, horrible exit, though!
Finally, LOTR is about the transfer of Middle-Earth to the dominion of Men, and the last withdrawal of direct contact from its creators and guardians. We know from the preface of FOTR that hobbits, if they survive at all, would wisely avoid contact with us Big People, clomping about the hedgerows. God only knows what they'd think about blood sports (wasn't a Took ancestor of Bilbo's the only one big enough to ride a horse?). It's not inappropriate, therefore, that these secondary human characters, in their relative complexity - their greyness between Light and Dark - come forward, especially in the movies. I don't see anything comparable in the SW mythos - Episode II was broadcast tonight and explanations of who's good and who's bad sound more like wonky position papers. It's notable that what Gandalf calls forth from Theoden, to save him from death-in-life, is his own free will reborn - it makes heroes of them both. Would we had a present analogue like that.