Stress is beginning to get to Atticus in Chapter 16 as the trial of Tom Robinson begins. Besides the important court case, Atticus has to deal with the everpresent addition of his sister, Alexandra, under the same roof. The two siblings had already verbally battled about the continued employment of Calpurnia, and Atticus had plainly told his sister that Cal was a member of the family and would be staying permanently. Everyone was still on edge about the lynch mob which had visited Atticus at the jail the night before. Alexandra was still complaining about their housekeeper; this time she was nagging Atticus about speaking so plainly about Mr. Underwood's dislike of Negroes in front of Cal. Alexandra thought Cal would spread the gossip to her friends.
"Everything that happens in this town is out to the Quarters before sundown."
The subtle change in her father, according to Scout
... was a quiet digging in, never outright irritation.
He becomes more stressed with the case and his sister is also in the house which causes some new problems. Both him and his sister argue about Cal and whether or not the kids should follow the roles of society. He was tired of all that was happening and just wanted justice to be given.