Chapter 15 of To Kill a Mockingbird details the scene when Atticus goes to the jail and sits out in front of Tom Robinson's cell in an effort to protect him from a possible lynch mob. When a mob actually does show up, the kids stay to help protect their father. Little did they know that Atticus did have backup by way of Mr. Underwood and his shotgun in the window of The Maycomb Tribune office building. From the children's perspective, as they are about to go home for the night, Scout says the following:
"It was long past my bedtime and I was growing quite tired; it seemed that Atticus and Mr. Underwood would talk for the rest of the night, Mr. Underwood out the window and Atticus up at him. Finally Atticus returned, switched off the light above the jail door, and picked up his chair" (155).
Atticus and Mr. Underwood must have been doing a few things in this situation. First, the mob had just left, but they could have waited for the children to go home and double-backed. Atticus and Mr. Underwood may have been discussing if they thought the Cunninghams were through for the night or thinking of coming back.
Next, Atticus did not go home immediately, suggesting that maybe he didn't want to leave Tom or Mr. Underwood alone if they still needed him. Also, with the Sheriff out running down another case, Atticus and Mr. Underwood may have been going over the names of the people in the mob. If the situation ever got out of hand, or came up again somehow, Atticus and Mr. Underwood would want to have a list of names ready for the sheriff. The text doesn't say if the sheriff was privy to Atticus and Mr. Underwood's plans that night, either, so they may have been discussing their side of the story so it would be polished and ready if the sheriff ever asked what happened.
Finally, Atticus and Mr. Underwood may have discussed how to handle the next day, which was the trial day. Not knowing Scout's effect on the Cunninghams that night, (if she had truly humbled them or not) they may have been discussing how to handle any possible craziness before, during, or after the trial as well.