In almost every play the characters are defined by their actions. Boy Willie's brash bull-headed nature is clear from even the very first moments of the play, when he loudly busts into the house at five in the morning. His stubbornness is shown by his refusal (until the very end of the play) to relent on his plan to sell the piano. Of course, his sister, Berniece, is just as stubborn as he is. She is just as deaf to his arguments as he is to hers. And then you've got Doaker, whose reasonable nature is shown by the way he constantly tries to make peace between the two bickering siblings.
As in all of his scripts, August Wilson includes some pretty detailed stage directions, which directly describe the characters. For example, he tells us that "Boy Willie is thirty years old. He has an infectious grin and a boyishness that is apt for his name. He is brash and impulsive, talkative and somewhat crude in speech and manner" (1.1.7). Yep, that's pretty direct and pretty darn descriptive. These sorts of stage directions are probably really helpful to the reader and maybe even to any production team who decide to produce The Piano Lesson. We should probably point out, though, that an audience watching the play would never hear these words.
Several of the characters' names give us clues as to who they are. Boy Willie, for example, is very boyish. August Wilson even tells us this directly in the stage directions, when he writes that Boy Willie has "a boyishness that is apt for his name" (1.1.7). You could say the Boy Willie almost acts like a child, throughout the play, as he stubbornly insists on getting what he wants. He's also constantly teasing everybody and messing around, giving him an even more childlike quality.
We should also note that Boy Willie isn't the first Boy Willie. He was named after his great grandfather Willie Boy, who originally carved the family's history into the piano. Also, Bernice is named after Willie Boy's wife Bernice, who was traded for the piano. It seems to us that, by naming Boy Willie and Berniece after their grandparents, Wilson more closely ties each character with the piano and with their family's past.