Final answer:
Atticus cuts off Heck Tate as he is about to reveal Boo Radley's involvement in saving Scout and Jem to protect Boo's privacy.
Step-by-step explanation:
In the given scenario, Atticus cuts off Heck Tate as he is speaking to Jem because Heck was about to reveal Boo Radley's involvement in saving Scout and Jem. Atticus wants to stop him from saying it in order to protect Boo's privacy and prevent him from becoming the center of attention. By keeping it confidential, Atticus ensures that Boo can continue living in seclusion without unwanted attention or interference.