Jack Nicholson, Matt Damon, and Leonardo DiCaprio make this movie great because they all have to play double lives. For instance Nicholson, who plays Frank Costello, an Irish mob boss in Boston, has to also be an FBI informant. Matt Damon, who plays Colin Sullivan, has to be a cop and Costello’s informant at the same time. DiCaprio’s character, Billy Costigan, grew up in a life of crime, and worked for Costello, but in secret he is a real undercover cop. Costigan and Sullivan are also sleeping with the same woman, which makes the subplots so intriguing.