In Harper Lee's To Kill a Mockingbird, character motivations drive the plot.
Atticus' motivations, which drive his actions, are based on being a man of honor. He is not racially prejudiced, and so he is willing to defend Tom Robinson as he would any many with a right to a fair trial. Atticus also is consistent: his sense of honor and integrity is something that drives him not only as a man and a citizen of Maycomb, but as the father he wants to be to his children in order to bring them up to be responsible, compassionate people.
Boo Radley is intrigued by the children. I believe he is motivated to want to be with them because they are interested in him, and they make him laugh. We see this when the kids are playing the "tire game" and Scout's tire rams into the side of Boo's house. Although it frightens her, she later admits to hearing a laugh outside. His fondness for the children and interest in their lives ultimately save them from Ewell's attempt to murder them.
Lastly, Bob Ewell is motivated by ignorance and hatred. He refuses to live with what he perceives as shame for his daughter's physical advances toward Tom Robinson, so he accuses Tom of rape. He is motivated by a sense of superiority in how he acts towards Tom and other blacks. And his verbal attack of Atticus on the street (when he spits on him), and ultimately his attack of Scout and Jem, are motivated by his deep hatred for those who do not support his malicious nature/ideas. He is the face of evil in the novel.
To best understand a character, it is important to understand what motivates him or her.