Build a movie plot around this: How does Sullivan learn Costello is and informant for the FBI? Sullivan learns Costello is an informant for the FBI by reading Queenan's diary.
The answer to this question is:
In voiceover, Irish-American mobster Frank Costello (Jack Nicholson) explains that he doesn't want to be the product of his environment, he wants his environment to be a product of him. Grainy historical film depicts violent riots in Boston during earlier generations. Costello, proudly Irish, unapologetically racist, explains that what annoys him about African Americans is their refusal to realize that they won't be given anything. Costello's belief is you have to do whatever possible to take what you want.Costello is in a small convenience store and collects protection money from the shop owner, who is visibly unnerved by Costello. Costello warns him to have more money next time. Costello asks the shop owner's young teenage daughter behind the counter if she's got her period yet. His attitude is nonchalant and all the more threatening for it. The daughter is both affronted and compelled by the powerful Costello and answers his rhetorical question. The shop owner doesn't show any emotion.A small boy, a young Colin Sullivan (Conor Donovan), is at the counter and witnesses Costello's interaction with the shop owner. Costello notices him and asks him if he's the son of a man Costello knows. He tells the store owner to give the boy two full bags of groceries and then presses some coins into the boy's hand. He tells the boy to remember he can get more money from Costello when he gets older.As Sullivan grows, Costello grooms him and steers him into a job as a police officer, where he can keep Costello informed. Sullivan completes the Police Academy and is immediately transferred to the State Police force. He is accepted into the Special Investigations Unit led by Ellerby (Alec Baldwin) which focuses on organized crime. The primary target of the unit is Costello. Sullivan (Matt Damon) is warmly welcomed onto the State Police force by Captain Queenan (Sheen) and Staff Sergeant Dignam (Wahlberg).Billy Costigan (DiCaprio), whose extended family has considerable ties to organized crime, also attends the police academy....