It depends on each reader's interpretation. As for the characters in the novel, it varies as well. Atticus might be surprised when Boo saves Jem and Scout from Bob Ewell, but in general, Atticus is not surprised to learn that Boo is a good, albeit terribly shy person. Even when Dr. Reynolds comes to check on Jem, he (Dr. Reynolds) speaks normally to Boo as if he were saying hello to anyone in the street. It is mostly the children (and any closed-minded adults who do learn about Boo) who are surprised to learn that Boo was just shy, which made it difficult to know that he was, in fact, a very good person who, in the end, turned out to be a hero.
Scout is probably most surprised of all. Jem had begun to figure Boo out a little earlier in the novel. In witnessing all that goes on in the adult world in Maycomb, Jem thinks that maybe it isn't Boo who's crazy; maybe it's that adult society.
If they’re all alike, why do they go out of their way to despise each other? Scout, I think I’m beginning to understand something. I think I’m beginning to understand why Boo Radley’s stayed shut up in the house all this time… it’s because he wants to stay inside. (Chapter 23)
At the end of the book, after Scout walks Boo home, she realizes Boo had been like the mockingbird: he'd kept to himself and had only done good things for others. Had Scout realized that this is what Boo was really like, she would have treated him differently:
Neighbors bring food with death and flowers with sickness and little things in between. Boo was our neighbor. He gave us two soap dolls, a broken watch and chain, a pair of good-luck pennies, and our lives. But neighbors give in return. We never put back into the tree what we took out of it: we had given him nothing, and it made me sad.