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Staff Sergeant Sean Dignam is a sergeant in the Massachusetts State Police. He worked in the Special Investigation Unit, in the undercover section with his boss Captain Oliver Queenan. Sean serves as the secondary late true main protagonist in Martin Scorsese's crime drama The Departed

He was portrayed by Mark Wahlberg.

Frank Costello's case[]

Billy Costigan recruitment[]

In 2006, Queenan and Dignam recruited Billy Costigan, a trainee with the Massachusetts State Police Academy, to their Special Investigations Unit. Billy was an ideal candidate due to his intelligence and his family‘s ties to organized crime. Queenan and Dignam ultimately intended for Costigan to infiltrate Frank Costello's crew. They arranged for Billy to be expelled from the academy and, to burnish his criminal credentials, to serve a four month prison term on concocted charges.

Microprocessors deal[]

In September 2006, Captain George Ellerby convened a meeting, in which the word was given to Dignam, who had the task of talking about the investigation in Costello's band, being the contact of the section infiltrated along with Queenan. Reports the news of the murder of Myles Kennefick, an employee of the MASS Processor, who was given the task by Costello of stealing 20 microprocessors, and handing them over for the purpose of selling them to Chinese mob. However he wants to move to Florida, and sell them, but he is murdered and the processors stolen by Costello. FBI agent Frank Lazio, asks if there is an infiltrator in the mob, but Dignam does not answer the question clearly, because he wants to keep the feds in the dark.

Queenan murder[]

After Queenan was murdered at the 344 Wash by Costello's men, Dignam attacked Sergeant Colin Sullivan, who had ordered the internal investigation to stalk Captain Queenan. Captain Ellerby intervened, who became the boss of Dignam, who dismissed Dignam for two weeks, with pay.

Sullivan murder[]

Dignam moments before murdering Sullivan.

While Sullivan returns to his house, in his apartment with the grocers, when he opens the door looks at Dignam, with wearing gloves and plastic hospital boots on his feet, ready to kill him by pointing the gun at his head. Dignam wanted to avenge the death of Queenan and Costigan, then Sullivan knowing that he could do nothing Sullivan said "Okay" before he was shot in the head. Dignam leaves the apartment, leaving the corpse there and the blood splashed on the corridor. In the film this isn't revealed, but probably to reach Sullivan's apartment, Dignam may have received the evidence showing that Sullivan was a mole for Costello, who were given by Costigan to Madolyn Madden, by Madolyn.


Despite his abrasive personality, he was extremely loyal to his boss Captain Queenan. Queenan is aware of his personality, and he knows that the new troopers are strange. He says "has a style of his own", he knowing everybody getting used to it. Captain Ellerby believes that he is normally a nice guy, but he knows him, and he knows he can be judged by meetings alone. The first time he meets Billy Costigan, he doesn't care about his impressions, he goes straight to the point of bringing to light his past, grown up in the bourgeoisie of Boston, but at the same time in South Boston, making him a perfect infiltrator. He never wanted to reveal to the other agents who the identity of the traced was, including federal officers. Dignam had suspicions that Colin Sullivan was Frank Costello's mole in SIU. Dignam is hard on Costigan, also threatening to eliminate his file and have him arrested like a simple Costello's soldier, making him angry, because at that time Costigan is very vulnerable as an infiltrator, with the fear that Costello could take him out to find out at any moment. On Queenan's murder, after assaulting Sullivan, Captain Ellerby dismisses him, but does not hesitate to say that he prefers to resign, as he will never want to work with the person responsible (indirectly) for the death of his boss and friend.


  • Originally Ray Liotta was cast as Dignam, but had to reluctantly decline, due to other commitments.
  • Mark Wahlberg turned down his role in The Departed multiple times. He finally changed his mind after Martin Scorsese convinced and persuaded him more than once to star in the film.
  • Wahlberg based his performance on the police officers who'd arrested him about two dozen times in his youth, and the reactions of his parents who had to come bail him out with their grocery money.
  • Wahlberg easily fell back on his native accent. Martin Scorsese joked it was so thick, they'd need subtitles.
  • Mark Wahlberg was one of the three Oscar nominees, along with Alec Baldwin and Vera Farmiga.
  • The Massachusetts State Police has not had staff sergeants since 1992.
  • Screenwriter William Monahan has stated that Staff Sergeant Dignam's first name is Sean.
  • When Billy Costigan meets with Queenan and Dignam under the bridge, Dignam's hair is messed up. From behind, his hair is perfectly straight.
  • When Costigan was at Boston Logan airport and spoke with Dignam on the phone, he is seen by Dignam's hand a stubtle, revealing that he is married.


  1. 1.0 1.1 George Ellerby: "How's your mother?"
    Sean Dignam: "Good. She's tired from fucking my father."