Tools of Characterization
Most of Billy Pilgrim's adventures are told in third person and without direct interference from the first-person narrator who starts and ends the book. At the same time, the narrator often tells us what to think of the people around Billy; the descriptions of the characters we get are pretty explicit and straightforward. We can find an example of this kind of direct characterization in the book's many descriptions of Roland Weary:
Billy and the scouts were skinny people. Roland Weary had fat to burn [...] He had so much energy that he bustled back and forth between Billy and the scouts, delivering dumb messages which nobody had sent and which nobody was pleased to receive. He also began to suspect, since he was so much busier than anybody else, that he was the leader.
He was so hot and bundled up, in fact, that he had no sense of danger [...] He was so snug in there that he was able to pretend he was safe at home, having survived the war, and that he was telling his parents and his sister a true war story—whereas the true war story was still going on. (2.24.1-2)
There is a lot of characterization going on in this passage. The contrast between Billy as skinny and Weary as fat seems to imply that Billy is vulnerable where Weary is spoiled. And Weary's layers of clothing show that he is (or at least, imagines himself to be) protected from the messed-up situation that he and Billy find themselves in.
The narrator also tells us directly that Weary "suspect[s] [...] that he is the leader" (though he clearly is not or should not be), that he has "no sense of danger," and that he is caught up in fantasies of what war is like because he is so good at ignoring the realities around him.
There are plenty of physical descriptions of Weary, but the real sense we get of his character comes from the narrator's outright judgment of him as a human being. The narrator calls Weary "stupid and fat and mean" (2.17.1)—it doesn't get more direct than that.
Let's look at the same passage we talked about above, in "Direct Characterization." It's true that the narrator is pretty direct about telling us what to think about Weary ("dumb" leaps to mind). At the same time, these descriptions of Weary do not exist in a vacuum. The narrator is not satisfied with telling us that Weary is "stupid and fat and mean" (2.17.1); he also has to show us that Weary is all of these things through Weary's actions.
Weary runs back and forth between the scouts and Billy for no reason except to pretend that he is the leader of their group. He makes up stories about the war because he has no patience to look around him and see what is actually going on. These actions are definitely those of a "stupid" man; as for "mean," it doesn't get much crueler than beating up a fellow American soldier behind enemy lines, as Weary does in Chapter 2, Section 33.
Thoughts and Opinions
This is a novel about fate, free will, and human suffering, so we probably shouldn't be surprised that characters have strong opinions about one or all of these things. Take, for example, Bertram Copeland Rumfoord, Billy's roommate in the Vermont hospital where Billy recovers from his skull fracture:
It was difficult for Rumfoord to take Billy seriously, since Rumfoord had so long considered Billy a repulsive non-person who would be much better off dead. (9.12.1)
We have spent the whole novel learning what Billy has gone through and how strong the narrator's regard is for the value of human life. Then along comes Rumfoord, who takes Billy's physical weakness as a sign that he is worth nothing, that he is a vegetable who should be killed for his own good. Rumfoord thinks strong men make the world and the weak deserve to die—exactly the kind of vicious view that Slaughterhouse-Five seems to have been written to reject.
But instead of telling us directly that Rumfoord is a dirtbag (as he does with Roland Weary), the narrator lets Rumfoord's own thoughts incriminate him. After all, it's tough to sympathize with anyone who thinks the main character of the book you're reading is "a repulsive non-person."