Jem and Scout had grown up hearing the wild stories and gossip about their unseen neighbor, Boo Radley, so their curiosity about what went on in the Radley house had been tempered. But when Dill comes to visit, he immediately becomes absorbed with the idea "to try to make him come out." Jem and Scout have already told Dill all the stories they have heard, so naturally he wonders "what he looks like?" Jem believes that the large footprints he has seen outside his house one morning belonged to Boo, and he thinks Boo must be a giant.
Boo was about six and a half feet tall... (Chapter 1)
Since rumors had it that Boo ate animals "raw," Jem believed that Boo's "hands were bloodstained." For unexplained reasons, Jem also believed that Boo resembled Frankenstein.
There was a long jagged scar that ran across his face; what teeth he had were yellow and rotten; his eyes popped; and he drooled most of the time. (Chapter 1)
The children also seemed to believe Miss Stephanie's story about Boo peeping in her window at night. According to her, "his head was like a skull. Old Mr. Radley "was a thin leathery man with colorless eyes... his cheekbones were sharp and his mouth was wide, with a thin upper lip and a full lower lip. Mrs. Radley was rarely seen except for when she came outside to water her plants.