When Gollum is about to lead Frodo and Sam into Shelob's Lair, Torech Ungol, this is how Tolkien describes his looks:
Gollum withdrew himself, and a green glint flickered under his heavy lids. Almost spider-like he looked now, crouched back on his bent limbs, with his protruding eyes. (4.8.87)
Gollum has decided to turn traitor and squash the Sméagol side of himself for good. He wants to lead Frodo and Sam to their doom, and his appearance reflects his choices as a character. He is withdrawn, his eyes glowing with a strange light. And most importantly, he looks "almost spider-like." Clearly, Gollum's looks in this scene foreshadow his choice to lead Frodo and Sam to Shelob the giant spider. Generally, in The Two Towers, if a character looks evil, he is probably planning something. But if a character looks wise or kind, he is probably working for the side of Good.
Speech and dialogue are such important factors to characterization in The Two Towers that Tolkien even uses them when he wants to show a single character fighting with himself. Take the example of Gollum. We know that Gollum is torn between good and bad impulses because his good and bad sides talk to each other all the time, even where characters like Sam can hear them.
As another example, Sam's way of describing Galadriel's beauty says more about Sam as it does about Galadriel: "Sometimes like a great tree in flower, sometimes like a white daffadowndilly" (4.5.128). Sam thinks immediately in terms of flowers because he is a gardener, and that influences how he thinks. And his use of the word "daffadowndilly," instead of the more formal and correct "daffodil," shows that he is of a lower social class than Frodo. Sam's way of talking tells us a lot about where he's from and what he's about, which in turn gives us some ideas of how he'll handle the hardships to come.
In The Two Towers, if you are a hobbit, you love smoking and jokes and generally cheerful living. If you are a Man of Westernesse (like Faramir or Aragorn), you enjoy serious scholarship, refinement, and being tall. If you are a man of Rohan, you love horses and hate orcs. And if you are an elf, you enjoy hanging out with trees and listening to the wind.
There are some individual variations in characterization, of course: both Boromir and Faramir are Men of Westernesse, but Boromir is much prouder and more ambitious than his brother. Still, two Men of Westernesse will share more in common with each other than they would with a dwarf or a hobbit. The kind of being you are makes a huge difference to your essential character traits in The Two Towers.