Harper Lee actually begins building the tension at the very end of Chapter 27 when Aunt Alexandra claims that
"... somebody just walked over my grave."
It was a "pinprick of apprehension"--a premonition--of the bad things to come on Jem's and Scout's journey from the the Halloween pageant.
The author creates a air of ominous eerieness in Chapter 28. It is a dark, moonless night, and the children are alone; normally, Atticus would have walked them to and from the school, but he was tired after spending a week in Montgomery, and decided not to attend the pageant. They pass the Radley house, which reminds them of the old superstitions in which they once believed. They still find the house "scary."
There are moments of foreshadowing: the wail of the mocker, the lack of visibility, the absence of a flashlight, and the surprising appearance of Cecil Jacobs, who "leaped at us," unseen in the darkness. After the pageant, the children decline a ride home, and the man warns them to "Be careful of haints." Jem cautions Scout several times to beware of falling, of losing her balance. She wears her ham costume home, restricting her movement and visibility.
Lee adds more tension when the children hear noises behind them on the trail. Nearly two pages are used as Jem and Scout try to convince themselves that the noises are but a dog or her "costume rustlin' " or Cecil preparing to scare them again. When the attack comes, Lee uses Scout--virtually blinded by the ham costume--to describe the action, and the reader is nearly as clueless to what is happening as Scout.
When the children finally reach home, Lee again slowly reveals the events--at one point the confused Atticus believes Jem to be Bob Ewell's killer--culminated by the climactic first appearance of Boo Radley, who is finally disclosed as the hero and killer of Bob.