In Chapter 7 of To Kill a Mockingbird, after Boo has repaired Jem's pants, the children find a ball of grey twine, which they leave in the knothole of the tree for a few days or so. But, after Jem and Scout remove it, little soap figures of themselves appear. Clearly, Boo Radley extends a part of himself tentatively to the children. For, in his lonely and alienated existence, he wishes to share some human contact. Unfortunately, after Jem and Scout write a thank-you note, Nathan Radley, Boo's brother, cements the hole, giving the excuse that the tree was sick. Ironically, a knothole in a tree indicates sensitivity and a willingness to give.
This scene from the first part establishes the credibility of Boo Radley's final actions. Having previously extended his love, then, in the last chapter, it does not seem so unusual for Boo to rescue the children from harm at the hands of Bob Ewell.