Only Boo knows for sure. We do know that Boo was watching over Jem and Scout on the Halloween night when they were attacked by Bob Ewell. Boo came to their rescue, killed Bob, and then carried Jem back to the Finch house. Boo was out and about on the night of Miss Maudie's house fire, stealthily placing a blanket upon Scout's shoulders. It was Boo's shadow which appeared on the night of the children's raid upon the Radley's back porch, and it was Boo who stitched up Jem's lost pants. Boo was peeking through the Radley curtains when Scout, being rolled in a tire, crashed onto the Radley steps.
... I had heard another sound, so low I could not have heard it from the sidewalk. Someone inside the house was laughing. (Chapter 4)
If you believe Miss Stephanie, she
... woke up in the middle of the night one time and saw him looking straight through the window at her..., (Chapter 1)
a sight that Boo could not have soon forgotten. Jem claims to have seen Boo's tracks, so Boo was probably witness to many nocturnal events that few other Maycomb citizens could have seen. As for the effect they had on Boo, it is clear that he developed a special interest in Jem and Scout, a bond strong enough to force Boo to risk his own life (and reclusiveness) to save their own. Boo made no other public appearances, and Scout "never saw him again" after that Halloween night. He had served his purpose as the children's guardian angel, and with Bob Ewell out of the way, Boo's work was done. He was still not ready to emerge into the "limelight" of the outside world. Perhaps the children were right: Dill decided that Boo didn't run off because "he doesn't have anywhere else to go"; while Jem determined that
"... Boo Radley's stayed shut up in the house all this time... because he wants to stay inside." (Chapter 23)