What details in chapters twenty-seven and twenty-eight add to the mounting tension before Jem and Scout are attacked?

In chapter 27, Scout reveals that three major things happened before Bob Ewell attacked the children. First, Mr. Ewell got a real job and lost it within a few days because he was too lazy to do anything at it. Of course he vocally and publicly blamed Atticus for the loss. Next, Judge Taylor was reading one Sunday night when he heard "an irritating scratching noise" near the back of his house (248). The judge thought it was his dog, but upon further investigation he discovered his back screen door cut and swinging open. Then he saw a shadow disappear into the night and sat with a gun across his lap for the rest of the evening. Finally, Bob Ewell started intimidating Tom's widow, Helen, as she was going to work for Link Deas. He would "chunk" at her--which probably means he spit after her--then he followed her down the road near his house, until Mr. Deas called him out on it and told him he'd have him arrested if he kept bothering his cook (249). 

The Halloween carnival is held at the school in chapter 28 and Jem escorts Scout around the Radley house, onto the school yard, and into the building before she must perform in her ham costume. Cecil Jacobs jokingly jumps out of the darkness and scares Jem and Scout terribly. This foreshadows Bob Ewell jumping out at them later that evening, but that won't be for a laugh. In fact, when Jem and Scout hear someone walking behind them after the Halloween carnival, they think it is Cecil Jacobs trying to scare them again. 

"I would show Cecil that we knew he was behind us and we were ready for him. 'Cecil Jacobs is a big wet he-en!' I yelled suddenly, turning around.

We stopped. There was no acknowledgement save he-en bouncing off the distant schoolhouse wall. . . It was unlike Cecil to hold out for so long; once he pulled a joke he'd repeat it time and again. We should have been leapt at already" (260-261).

When the kids realize that it must not be Cecil following them, terror really starts to take over. They hear someone's cotton pants rubbing together and the sound of a man's steps. When those steps increase with speed, Jem tells Scout to run. It's at this point, under the Radley tree, that the children are attacked by Bob Ewell.