Since there is not much physical description or even speech, Conrad develops most of his characters through action. As the protagonist, Marlow receives a great deal more interior consistency than most of the other characters. The accountant, the manager, the brickmaker, and even Kurtz show a great divide between what they say and what they do. Often, their actions are far more indicative of their true character than their speech.
We know a lot about Marlow because of what the peripheral, nameless narrator tells us. "Marlow was not typical" (1.9), he explains. Uh, yep. Actually, we probably could have figured that out without the direct characterization. But thanks for the boost!
Did you notice that no one around here seems to have a name? Besides Marlow and Kurtz, that is. Everyone else just has a title/profession: the accountant, the harlequin, the manager, the Intended. It could be that this illustrates the dehumanization of men in the wilderness of the Congo. If this is true, then what's so human about Marlow and Kurtz?
Since Marlow is the narrator, we get mostly his speech and thoughts in his specific vernacular, which is very turn-of-the-century British. As a general rule, only the white people speak and their speech is given in standard English. Some of the characters—namely the brickmaker and the harlequin—have rather erratic speech, like this little bit from the harlequin:
'Brother sailor... honour... pleasure... delight... introduce myself... Russian... son of an arch-priest... Government of Tambov... What? Tobacco! English tobacco; the excellent English tobacco! Now, that's brotherly. Smoke? Where's a sailor that does not smoke?" (2.35)
One thing to note: the harlequin is Russian, and we have our sneaking suspicions about the brickmaker being Jewish. (Check out his "Character Analysis" for some thoughts on that.) To Conrad, that would mean that neither guy is exactly "white." They may have white skin, but they're still "other"—just like their speech.
For the most part, black slaves and native Africans don't speak, at least not English (which to Conrad means, not at all). In some instances they communicate nonverbally, through ritualized gestures or loud emotional cries. Does this count as communication to Marlow? To us?
Occasionally, a black slave (such as the manager's boy) or a cannibal speaks in English to communicate vital information to Marlow, but he never uses proper English—which, of course, emphasizes that the speaker can't fully function in a white man's world.
A good example is the super-famous "Mistah Kurtz—he dead" (3.45). (Why is this super famous? T.S. Eliot used it as the epigraph to his poem "The Hollow Men," which is kind of a big deal—as in, articulates the literary viewpoint of an entire generation.)