Direct characterization is when the text just gives you information, such as "Jack is a jerk." (Or, “everyone around me is a phony.”) And Holden says things like that all the time—but Holden is a character himself, so any information we get through him is actually indirect. It passes through his filter and comes out fictional. There is no unbiased and omniscient narrator to give us direct characterization; there's just Holden. As such, much of what he says about other people is really as much of a commentary about himself as it is anything else.
Physical descriptions are a pretty good indication of character in The Catcher in the Rye. Stradlater is big, athletic, and good-looking; accordingly, he's a player. Ackley is pimply and un-hygienic, and everyone dislikes him. Holden is small for his age, and this seems to be a source of insecurity for him; he always tries to act older. Phoebe is cute adorable and she's a joy to be around. See a pattern here?
Or, do you see a pattern of how Holden seems to see the world?
Want to know something about Holden? You’re in luck: you have pretty much unfiltered access to his entire brain. We get to hear what Holden thinks about everything. He thinks Jesus' disciples were useless, that the movies are phony, that most girls are dopey but still attractive, that getting a job and being an adult sounds like just about the worst racket he's ever heard, etc. Holden is defined by his impressions—and the way he presents those impressions to us.
We already know what Holden's real speech sounds like—it's the voice he uses in his narration to us. But check out how he speaks when he's talking to people like Mr. Spencer. He says "sir," holds himself back from digressions, and tries his best to placate and please. That's a far cry from "You'll probably want to know […] all that David Copperfield kind of crap, but I don't feel like going into it" (1.1).
This is how Holden describes his initial interaction with Sunny and how he tries to talk to women, or bartenders, or adults in general whom he wants to impress. Holden tries to present an older persona by using what he considers socially adept phrases ("Allow me to introduce myself," "How do you do," and "Come in, won't you?"). Given the reaction of the women in the Lavender Room, we're not sure how convincing he is. But he gets points for trying.
Holden definitely has an ear for little kids' speak, which "kills" him. We're looking in particular at the scene in the museum in Chapter Twenty-Five, when one of the two "bruddas" asks where the "toons" (“tombs”) are (25.22). Holden understands exactly what the kid's saying, which reinforces what we already knew about Holden and his ability to interact with children.