It's all about actions in Prince Caspian. A quick look at our two dwarf friends, Nikabrik and Trumpkin, is all it'll take to prove this point:
And sure enough, Trumpkin equals our righteous dwarf, and Nikabrik our nasty one. But it doesn't stop with the dwarf characters. You can look at any opposing pair of characters—Caspian/Miraz or Peter/Edmund or Lord Glozelle/Sopespian—for examples. If you compare their actions, you'll see how the actions make the characters.
Mark Twain once said, "Clothes make the man. Naked people have little or no influence on society" (source). Too true, and it seems to be that way everywhere, as clothing makes the man in Narnia as well.
Case in point: when Peter and Edmund are in their school clothes, Trumpkin doesn't consider them to be much more than children; certainly not warriors. But then they change into armor, "jingling in their mail, and already looking and feeling more like Narnians and less like school-children" (8.24). The change in clothes—not to mention being trounced in swordsmanship—quickly changes Trumpkin's mind on the subject.
We might add accessories to this list, too, because what's an outfit without accessories? (Not an outfit, that's what.) For example, Susan's horn is given to Caspian. As a relic of Old Narnia, the horn coming into Caspian's possession signals his acceptance of the older ways. When blown, it calls the Pevensie children to Caspian's aid, making it both practical and fabulous.
The talking animals and walking trees have that oooooh supernatural feeling, but they are characterized as being not-so-supernatural. That is, they tend to be closer to nature and the old ways. On the other hand, characters of a more human variety (dwarves and, obviously, humans) tend to be characterized as further from nature.
As Trufflehunter the badger notes, "'You Dwarfs are as forgetful and changeable as the Humans themselves. I'm a beast, I am, and a Badger what's more. We don't change" (5.64). And let's not forget Glenstorm the Centaur, who uses the stars to foretell that the time for war has come (6.21). We are, of course, counting his horse-half here more than his human-half.
But what about our human brethren? Well, when Lucy walks through the woods, she wants the trees to talk again (talk to her as the stars "talk" to Glenstorm). But at that moment, she feels "she had just missed something: as if she had spoken to the trees a split second too soon or a split second too late, or used all the right words except one" (9.9). And don't even get us started on the Telmarines. They're just plain afraid of trees, so there won't be much dialogue there.
All is not lost for our humanoid characters, though. Lucy may not be a talking squirrel, but she eventually does reconnect with Aslan and the talking trees and, in the end, nature. It just takes her that bit of extra effort.