He lives by example. His behavior is consistent, he could never be labeled a hypocrite. He influences others by being so different from most everyone else in the town, so he stands out. Being a lawyer, he is intimately involved with town affairs, so he’s in the public eye. He can’t help but be noticed, so in an empirical sense, he’s influential simply because he is involved in the town. I think it is Scout who recalls what Miss Maudie said that Atticus is the same at home as he is in the courthouse. Atticus’ behavior is consistent, he’s always in the public eye and he is the moral backbone in Maycomb. He’s chosen to represent Tom because they know he will do what most of the townspeople could not; stick to his moral principles regardless of how others perceive him.