After finding an entire attic full of books and magazines, Montag's hands seem to move of their own volition and he steals a book. As they are preparing to burn the books, Montag feels irritated because the old woman is still there; usually, the police remove a book-owner so the firemen only have to worry about the books. The old woman refuses to leave her home, even when Beatty warns her that they will ignite the kerosene:
The woman on the porch reached out with contempt for them all, and struck the kitchen match against the railing.
People ran out of houses all down the street.
(Bradbury, Fahrenheit 451, Google Books)
This shows two things:
First, people still fear fire, even though their houses have been entirely fireproofed. The removal of fire as an accidental danger is one of the factors that leads to firemen becoming enforcers of book law instead of rescue workers, but humans have an innate respect for the harm that fire can cause, and even though their houses are in no danger, they are afraid the fire might spread.
Second, they are interested in watching the spectacle. This shows the alienation and moral collapse of the future society; without boundaries and ethics (learned in part from differing opinions, which are rare without books) people take joy in any emotional outlet. Instead of having concern for the old woman and indignation at the government's power, people get a brief thrill out of her death, and a story they can tell later.