David Mamet's play American Buffalo can be said to be a character study of three different types of men. The personalities of Teach, Don and Bobby are revealed under the pressure of Don's confined and cluttered junk store.
Teach appears to be the dominant figure of the trio, vivid, volatile and eventually violent. He wants to be the leader of the pack but is always in competition with Don for the position. Teach respects Don more than he likes him; (there is also a sense that he physically fears the store owner). He despises Bobby, and he resents -- and is, possibly, jealous of -- Don's strong emotional tie to the troubled young man. The chip on Teach's shoulder is considerable; he is definitely, in his own estimation, "a man more sinned against than sinning" (cf. Shakespeare's King Lear). He is a bundle of nerves and insecurities. His entrance into the play, an explosive tirade against a woman named "Ruthie," reveals far more about himself than it does about his female nemesis.
Don is seemingly mellow, but he is the true backbone of the group. The reader senses that he, not Teach, is the one who would not crumble in the crucible, not faint under fire. He likes his buddy Teach but doesn't respect him beyond someone who can help him better his own position financially. Don seems to tolerate him - wary of, but also amused by, his antics. Don's obvious love and concern for Bobby is honorable, at times even moving. He is aware of the young man's problem with drugs but refuses to focus on it and doesn't allow Teach to. Don is "a stand-up guy." (Perhaps if there were ever honor among thieves, he would be its poster-child.)
Bobby is clearly the youngest and least stable of the three. There is little explicit reference to his drug addiction, but Teach maliciously touches on it now and then, (despite Don's protests). He loves and respects Don and wants badly to please his mentor. (There is the slightest hint of a homo-erotic involvement between the two, but Mamet never underscores it.) Bobby is, in a way, the catalyst to the action; his instability and failings push the threesome to its crisis. He always means well, but his circumstances (particularly, perhaps, his substance abuse) do not allow him actually to do well.
Teach, Don and Bobby are a combustible combination. Their diverse personalities, crystallized under pressure in the claustrophobic setting of the crowded basement junk store, clearly clash. Teach's bravado, Don's paternalism and Bobby's instability lead the three men to what feels like an inevitably violent conclusion.