Physical appearances often give us an immediate clue to "what lies beneath" in this book. When we meet Pablo, for example, we learn from the description that he's grizzled, scarred, a bit oddly shaped, with close-set narrow eyes, and has a somewhat hostile look on his face. A shady, unpleasant character.
Pilar is monumentally large, strong and thick, with a warm brown face that looks like a "model for a granite monument." Her looks reflect her courage, strength, good humor, and larger-than life vitality.
Robert Jordan has a physique which suits his restraint, his toughness, and his weathering by the war: he's tall, lean and muscular – presumably rather chiseled – with rough, sun-streaked hair and skin burnt by the sun and wind. Plus those sharpened, clear eyes.
Maria's eyes, on the other hand, are "hungry, young, and wanting." A head of cropped hair, which people seem to agree mars her beauty, testifies to the horrible events of her past which still haunts her.
Through Robert Jordan's reactions and the commentary of other members of the band about each other, we get plenty of direct information. Robert Jordan thinks Pablo is a bad egg the moment he sees him, and tells us so, and every other character basically says the same. Anselmo, on the other hand, is always a "very good man" (at one point even the narrator just straight up says it), and El Sordo's a courageous, trustworthy, and all around capital guy. By the end of the second chapter, we know from other characters that Pilar is brave, barbarous, and at times very gentle. About Robert Jordan himself, the narrator does give us some important direct descriptions, telling us right at the outset, for example, that he doesn't really value his own life and that his devotion to his cause is strong.
"That we blow up an obscene bridge and then have to obscenely well obscenity ourselves off out of these mountains?" (3: 127) Thanks for that, Agustín – can we even call that a sentence? There are several potty-mouths in For Whom the Bell Tolls, and a lot of interesting cusses, though Hemingway never writes them out: he always replaces them with "obscenity" or "unnameable" or whatnot (a few times, he does let very foul words in untranslated Spanish).
Swearing is one of the major ways in which characters are given color, and personality. It defines Pilar and Agustín, and the more cynical way Robert Jordan swears also contrasts him with his more exuberantly obscene Spanish friends. Love of cursing in general is meant to be characteristically Spanish.
Hemingway also lends that "Spanish-ness" to his characters' language by using really awkward straight translations into English: lots of thee's and thou's, and words which mean something different in normal English than Spanish (for example, "molest," which means bother in Spanish and is a much more everyday word). Every so often, a character will also break into a regional dialect, as Anselmo does when he curses Pablo out at the very beginning of the book.
If somebody steals your detonators and runs away with them, thereby screwing your mission over and dooming you and your friends to die, chances are he's a jerk. If, even after he returns, he kills the people he's recruited to help him, he's still a jerk, though he's a jerk on your side.
The actions of characters in For Whom the Bell Tolls reveal a lot about them. As it's a war novel, the most common traits revealed by action are bravery and brutality: Pilar's bravery is evident from her standing up to Pablo and willingness to go ahead with the mission, as well as her military performance, while Pablo's brutality is shockingly clear from her story about the fascist massacre in their town. Resolve is another big one: Anselmo's the guy who stays put in the snowstorm, even when it gets really bad, because he doesn't want to disobey Robert Jordan. If you want courtesy, think of El Sordo's bringing a bottle of whiskey specially for Robert Jordan from La Granja, this in the middle of a war.