At the end of the first section of Fahrenheit 451, "The Hearth and the Salamander," Montag makes a well meaning but impulsive decision to tell Millie about the books he has in the house. Millie reacts as if she's being attacked.
Mildred backed away as if she were suddenly confronted by a pack of mice that had come up out of the floor. He could hear her breathing rapidly and her face was paled out and her eyes were fastened wide.
In "The Sieve and the Sand," Montag decides to read poetry to Mildred and her friends. The reading itself is the impulsive move. Here is the narrative description of their reactions:
Mrs. Phelps was crying.
The others in the middle of the desert watched her crying grow very loud as her face squeezed itself out of shape. They sat, not touching her, bewildered with her display. She sobbed uncontrollably. Montag himself was stunned and shaken.
Faber scolds Montag repeatedly for this impulsive move because he wanted to pursue Montag's education in secret. However, Montag's awakening is so dramatic and affects him so deeply that he can't resist acting out.