Bradbury sometimes just comes out and tells us what characters are like. He particularly likes to do this in the interchapters, when he's describing a group of people. For instance, in "The Old Ones," the narrator describes one group who came to Mars as "the dry and crackling people, the people who spent their time listening to their hearts and feeling their pulses and spooning syrups into their wry mouths" (2). That's a pretty direct characterization of old people.
(It's also not a very nice one. Yikes, Bradbury. Don't you have grandparents?)
When we're in a story told from a particular character's point of view, Bradbury does a lot of characterization through thoughts and opinions.
Like, Ylla watches her husband reading and "quietly wished he might one day again spend as much time holding and touching her like a little harp as he did his incredible books" ("Ylla," 10). That little word "quietly" tells us a lot about her: she's humble, maybe a little insecure, not going to make a fuss, and has probably been feeling like this for a long time.
Or Stendahl, who gloats over his plan to kill a bunch of government censors: "How he would savor this in his old age. This paying back of the antiseptic government for its literary terrors and conflagrations. Oh, how the anger and hatred had grown in him through the years. Oh, how the plan had taken a slow shape in his numbed mind" ("Usher II," 95).
So, we get a pretty clear sense that this is one angry dude—but also, check out the slow way these sentences take shape through the repetition of "how" and the use of fragments. The very way he speaks (or thinks) is a clue to his slow, elaborate revenge.
Want a cheat sheet to characters in The Martian Chronicles? Just make yourself a little chart to separate characters who shoot people (Parkhill, Spender, Yll) from ones who don't (Tomás Gomez, Wilder, Benjamin Driscoll).
The "bad guys" can't deal with difference. Parkhill is afraid of Martians, but Gomez likes strangers. Spender prefers Martians to humans, but Wilder seems to want compromise. Yll is afraid of losing something (his wife, Ylla), while Benjamin Driscoll wants to add something positive to the world (his trees).
Bradbury loves to use contrast to show us something about characters. For example, in "The Off Season" Sam Parkhill is panicky and deluded, while his wife Elma remains a calm observer of reality. Biggs and Spender, in "—And The Moon Be Still As Bright," are opposed in their relationship to Mars: Biggs messes it up while Spender wants to save it. And in "The Fire Balloons," flexible Peregrine is paired up with the inflexible Father Stone.
Bradbury will also put characters in different stories into the same situation. For instance, Walter Gripp and Dr. Hathaway both think they're the last man on Mars, but they have radically different attitudes about that. Or take the scenario of "human meeting a Martian": when Parkhill meets a Martian, he shoots first and—well, actually, never asks questions.
When Tomás Gomez meets one, though, he smiles. And what might be even more surprising is that he "listens" ("Night Meeting").