What does Cecil Jacobs do before the pageant which sets up the incidents that occur afterward?

While Jem and Scout were on their way to the Halloween pageant, Cecil Jacobs sneaked up on them and scared them in the dark of night. 

Cecil had ridden safely to the auditorium with his parents, hadn’t seen us, then had ventured down this far because he knew good and well we’d be coming along. He thought Mr. Finch’d be with us, though.

Walking home, Jem heard someone's footsteps behind them. Having just been frightened by their friend the same evening, it was natural for the children to assume that Cecil was just trying to scare them a second time. 

“It’s just old Cecil,” said Jem presently. “He won’t get us again. Let’s don’t let him think we’re hurrying.”

As a result, Jem and Scout slowed their pace to lure Cecil in, allowing their stalker, who turned out to be Bob Ewell, to catch up to them. They only realized they were in danger when the footsteps they heard started running toward them, but they didn't sound like they belonged to a child.

Shuffle-foot had not stopped with us this time. His trousers swished softly and steadily. Then they stopped. He was running, running toward us with no child’s steps.

Had Cecil not have pranked Jem and Scout, they would probably have been much more cautious of the situation and more aware of the danger they were in.