Here are three quotes from Chapter 1 about what Scout and Jem believe is Boo's physical appearance and what they think are his habits:
"People said he [Boo] went out when the moon was down, and peeped in windows. When people's azaleas froze in a cold snap, it was because he had breathed on them. Any small crimes committed in Maycomb were his work."
"The Radleys, welcome anywhere in town, kept to themselves, a predilection unforgivable in Maycomb. They did not go to church, Maycomb's principal recreation, but worshiped at home."
"Boo wasn't crazy, he was high strung at times."
Boo is a figure of mystery and misunderstanding, and because he is different from other people in Maycomb, he becomes a kind of scapegoat. He is blamed for everyday happenings that are not his fault, such as flowers freezing in the cold. As he and his family keep to themselves, perhaps fearing that the townspeople wouldn't understand Boo, they become figures of even greater mystery. The town doesn't understand why they isolate themselves and act unsociably, even on Sundays--the traditional day for paying calls. Boo is described as "high strung" after he allegedly stabbed his father with some scissors; in reality, he is a kind man. It takes the entire book for Scout and Jem to realize how giving and wonderful Boo truly is.