What is it about Tim Johnson’s appearance and behavior that makes Jem think that the dog has rabies? How does Calpurnia respond when she sees the dog?
Tim Johnson is walking strangely and seems to be acting abnormally. He's walking very slowly and twitching. Everyone in the town knows Tim Johnson well, and so Jem is easily able to see the change in the dog's behavior because he knows what Tim's "normal" behavior is like.
The kids tell Calpurnia and, after looking at Tim Johnson, she immediately calls Atticus to let him know there is a rabid dog on the loose. She then calls the town operator to have her call and warn each of the neighbors, and she runs to the Radley house and yells at the house to warn them. She is very proactive in making sure that anyone who could come in contact with Tim Johnson stays away.
Atticus comes directly home, gets his gun, and much to the surprise of Jem and Scout, shoots Tim Johnson. Although Atticus shoots the dog and "saves" everyone, it is really Calpurnia who kept everyone from harm by warning them.
In chapter 10, Jem and Scout spot an old dog named Tim Johnson staggering down the street. Jem immediately notices that something is wrong with the dog and tells Calpurnia to come look at it. Using animated gestures, Jem explains to Calpurnia that the dog is twitching, acting erratic, and losing its balance as it slowly walks down the street. When Calpurnia sees Tim Johnson staggering, she immediately recognizes that the dog has rabies. Calpurnia quickly runs to the telephone and explains the situation to Atticus. She then tells the operator to inform the community members to stay inside their homes before she runs to tell the Radleys. Fortunately, Heck Tate and Atticus arrive in time before anyone is attacked by the rabid dog. Heck Tate gives Atticus his gun and Atticus kills the dog in one shot.