This week in the magazine, Sarah Stillman writes about people conscripted by the police to work as confidential informants, often at great personal risk. Here Stillman joins Evan Ratliff, who has also written about confidential informants, in a conversation with Nicholas Thompson about why law enforcement relies on C.I.s, and what kinds of oversight and controls are needed to ensure their safety. Also, tips from Paul Simms on how to win a conversation.