Character Clues
Character Analysis
Direct Characterization
Subtle characterization is not generally Melville's thing; he'd rather just tell you flat out what a character is like. So, Amaso Delano is "a person of a singularly unmistrustful good nature" (3.4), while Turkey (in the afternoons at least) has "a strange, inflamed, flurried, flighty recklessness of activity about him." (2.6)
Speech and Dialogue
The most famous bit of characterization in this book, and one of the most famous in all American literature, is Bartleby's catch-phrase, "I prefer not to." That tells you everything you need to know about Bartleby (or everything you would prefer not to know, as the case may be.)
Actions
In a number of cases in the stories, characters are revealed to be untrustworthy or dangerous or evil through acts of violence. The Lightning-rod man's attack on the narrator shows once and for all that the salesman is a fraud and a bad guy. Bannadonna's murder of the workman shows pretty much the same thing about him in "The Bell Tower". And the hermit Oberlus' nastiness is demonstrated by the way in which he keeps attacking people. (On the other hand, when Amaso Delano's men assault the slave ship, they're presented as being valiant. So it all depends.)
Physical Appearance
In "Benito Cereno", Melville often uses phsysical appearance to show you that Don Benito is not what he seems, or that the characterization you're getting of him from Delano is off or wrong somehow. When Babo cuts Benito shaving, for example, Delano observes that Benito has a "terrified aspect." (3.266) You don't know what he's terrified of, precisely, or why, but it lets you know that there's something you don't know. Appearance here reveals that there's more to Benito than appearance.