Donnie Brasco (1997)

Aging gangster Benjamin “Lefty” Ruggiero is introduced to a jewel thief named Donnie Brasco. Donnie impresses Lefty by threatening a diamond dealer whom Donnie suspects of selling Lefty a fake ring. Lefty teaches Donnie the rules of the Mafia and introduces him to several “made men” including Dominick “Sonny Black” Napolitano, and Nicholas Santora, as well as caporegime Alphonse “Sonny Red” Indelicato to whom Lefty owes money and is disliked by Sonny Black.
Donnie Brasco is actually Joseph D. Pistone, an undercover FBI agent. His wife hates his job, and the couple have heated arguments throughout the film. At home, Joseph’s behavior becomes more and more like the criminal he pretends to be.