One example of people not always being as they seem in To Kill a Mockingbird is Atticus Finch. His character is presented as a man who is cautious and careful, very laid-back and non-confrontational. However, Finch is known as the best shot in the county as demonstrated when he was called to shoot the rabid dog. He willing took the case of the black man accused of raping a white girl, and he handled the mob outside the jail who came for the sole purpose of hanging the black man. He definitely demonstrated more courage and strength than the author demonstrated in the beginning of the story.
This question touches on a wider theme that occurs very frequently in a variety of different texts. It can also be called appearances vs. reality, and is used to indicate the ways in which people place trust in appearances often more than they do in reality. The classic example of this in Lee's famous text is of course Boo Radley and the way that the children at the beginning of the novel clearly see him as some sort of bogeyman or terrible creature that never comes out during day time. The games they play all are based around this myth, and it is clear that they have taken and accepted the rumours and gossip they have heard from other residents and built them into an elaborate fantasy of their own. However, by the end of the story, Scout has learnt the important lesson of understanding the perspective of others. Note what she says when she stands on the Radley porch:
Atticus was right. One time he said you never really know a man until you stand in his shoes and walk around in them. Just standing on the Radley porch was enough.
She has been able to move beyond her prejudices and the way that things seem to find out the true reality and act accordingly. This of course has important consequences when the racial element of this text is considered.