Some of the characters in Charlotte's Web are just what you'd guess they'd be: the goose is a silly goose, and the rat is, well, a rat. We think rats are pretty sneaky and grimy, and Templeton is just that. So knowing that Templeton is a rat helps us to understand what he's like.
But some of the characters surprise us. Wilbur is a pig and he sure does like to pig out on his slop. Yet he's also willing to give up some of his food to help his friend. See what we mean? The fact that Wilbur is a pig both tells us about him and sets us up for some shockers.
You know what they say: you are what you eat. And that saying definitely holds for the characters in Charlotte's Web. As a pig, Wilbur likes to pig out on slop. Slop is like leftover stew: dump everything but the kitchen sink into some milk, and Wilbur will be happy as, well, a pig.
We learn about other characters based on what they chow down on, too. Check out Charlotte: unlike Wilbur, no one brings her food. So she has to catch her own flies, which makes her one resourceful gal. And then there's Templeton. He gets Wilbur's leftover leftovers. That makes him the lowest on the food totem pole. He also happens to be lowest on the social ladder in the barn. Coincidence? We think not.
Sometimes our narrator just tells it like it is. Check out the description we get for Templeton:
"The rat had no morals, no conscience, no scruples, no consideration, no decency, no milk of rodent kindness, no compunctions, no higher feeling, no friendliness, no anything. He would kill a gosling if he could get away with it—the goose knew that. Everybody knew that." (6.20)
The narrator gives us the skinny on this fat rat—no insinuation or implication, just the facts. We're not left guessing with this one.