Jem and Scout have gotten used to living near the Radley House, and though it
... was inhabited by an unknown entity the mere description of whom was enough to make us behave for days on end...,
it is the arrival of Dill that really sparks the intense interest that they display throughout most of the first eight chapters of the novel. Jem and Scout have heard all of the rumors concerning Boo's nocturnal habits and, naturally, they pass them on to Dill. The summers in Maycomb were boring ones, and the children had to find ways to while away their time. Dill had seen the movie Dracula, and the stories about Boo must have charged his imaginative nature.
The Radley place fascinated Dill... it drew him as the moon draws water, but it drew him no nearer than the light-pole on the corner, a safe distance from the Radley gate. There he would stand... staring and wondering.
Dill could not get Boo off his mind, and he would "Wonder what he looks like," until, finally one day, he proposed an idea that Jem and Scout considered near suicidal.
"Let's try to make him come out," said Dill. "I'd like to see what he looks like."
It was a combination of the long, slow summer days; the knowledge of the unseen "malevolent phantom" who lived inside; Dill's imagination; and the curiosity of the children that caused their intense fascination with the Radley House.