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...

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...

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.