I'm afraid the previous post has its facts wrong about the good Doctor Reynolds. Reynolds is one of Atticus's supporters who comes to his home to warn him about the possible trouble that may occur when Tom Robinson is moved to the Maycomb Jail. He is NOT one of the lynch mob who confronts Atticus the next day. Reynolds is one of the more enlightened citizens of the town, an educated professional who, like Atticus, accepts trade goods for his medical services. Reynolds isn't really an essential character in the chapter; it is primarily author Harper Lee's way of introducing him for his more important role at the end of the novel when he treat Jem's broken elbow.

Newspaper editor B. B. Underwood is an interesting man. Named after the inept Confederate General Braxton Bragg "in a fey fit of humor" by his father, Underwood does his best to live down his name, drinking from his constant companion: a jug of cherry wine. According to Atticus, Underwood "...despises Negroes. Won't have one near him." Yet, as the town's journalist, he does believe in justice and fair play, and he is later found to be standing guard over Atticus and the lynch mob with a shotgun. Racist though he may be, he is willing to put his life on the line for Tom if necessary. Later, Underwood laments about Tom's death, calling it akin to the "senseless slaughter of songbirds by hunters and children." 

Chapter Fifteen is where a group of angry men confronts Atticus outside the Maycomb jail. Among the crowd are a number of prominent people, including Dr. Reynolds, which makes the important point that people act differently in groups than they do individually. Dr. Reynolds, a family doctor to virtually every white person in the town, has joined a mob that is trying to break in the jail to get Tom Robinson. 

After a very tense showdown, in which Scout breaks the tension by singling out a member of the mob for conversation, thus depriving them of the anonymity of the group, Atticus and his family learn that Braxton Underwood, the publisher of the town newspaper, was covering them out of his adjacent office window the entire time. Not everyone in Maycomb, the reader is reminded, is subject to the mob mentality. The irony is that Underwood is a self-confessed dyed in the wool racist.