Ash has seen this movie.
To take down South Boston's Irish Mafia, the police send in one of their own to infiltrate the underworld, not realizing the syndicate has done likewise. While an undercover cop curries favor with the mob kingpin, a career criminal rises through the police ranks. But both sides soon discover there's a mole among them.
| Billy | Leonardo DiCaprio |
| Colin | Matt Damon |
| Costello | Jack Nicholson |
| Dignam | Mark Wahlberg |
| Queenan | Martin Sheen |
| Mr. French | Ray Winstone |
| Madolyn | Vera Farmiga |
| Brown | Anthony Anderson |
| Ellerby | Alec Baldwin |
| Cousin Sean | Kevin Corrigan |
| Barrigan | James Badge Dale |
| Fitzy | David O'Hara |
| Delahunt | Mark Rolston |
| Lazio (FBI) | Robert Wahlberg |
| Gwen | Kristen Dalton |
| Govenor | Thomas B. Duffy |
| Uncle Edward | Richard Hughes |
| Realtor | J.C. MacKenzie |
| Billy's Aunt | Mary Klug |
| Mrs. Kennefick | Peg Holzemer |
| Triad Boss | Robert 'Toshi' Kar Yuen Chan |
| Pakistani Proprietor | Gurdeep Singh |
| Providence Gangster #1 | Armen Garo |
| Providence Gangster #2 | John Cenatiempo |
| Kneecapped Bankrobber | Joseph Riccobene |
| Detective #1 Tailing Queenan | Billy Smith |
| Translator | Lyman Chen |
| Detective #1 - Colin's Unit | Kevin P. McCarthy |
| Detective #2 - Colin's Unit | Chris Fischer |
| Man Glassed in Bar | Brian Smyj |
| Older Priest | William Severs |
| Younger Priest | Larry Mitchell |
| Police Camera Tech | Anthony Estrella |
| Police Cell Phone Tech | Andrew Breving |
| Darlene the Secretary | Tracey Paleo |
| Man Killed by French | Douglas Crosby |
| Woman at Bar #1 | Dorothy Lyman |
| Woman at Bar #2 | Audrie Neenan |
| News Anchor | Frank Mallicoat |
| Billy's Mother | Paula DeMers |
| Young Colin | Conor Donovan |
| Carmen | Amanda Lynch |
| Woman at Opera | Sallie Toussaint |
| Bookie Harassed by French | Patrick Coppola |
| Jimmy Bags | Mick O'Rourke |
| Sister Mary Theresa | Deborah Carlson |
| Queenan's Secretary | Nellie Sciutto |
| Other Prisoner | Peter Welch |
| Chinese Government Man | Henry Yuk |
| Costello's Crew #1 | Dennis Lynch |
| Costello's Crew #2 | Michael Byron |
| Costello's Crew #3 | William Lee |
| Luncheonette Proprietor | John Rue |
| Man in Costello's Bar | Peter Crafts |
| Priest w/ Young Colin | Joseph P. Reidy |
| Priest at Billy's Funeral | John Farrer |
| Jimmy Pappas | Paris Karounos |
| Detective #2 Tailing Queenan | Brian Haley |
| Detective #3 Tailing Queenan | Terry Serpico |
| Detective #4 Tailing Queenan | Jay Giannone |
| Lynn Detective | John Polce |
| Drill Instructor #2 (voice) | Bo Cleary |
| Gang Member with Mr. French | David Conley |
| Chinese Delivery Man | Victor Chan |
| Executed Woman | Jill Brown |
| French's Wife | Sarah Fearon |
| Woman Doctor | Denece Ryland |
| Crack House Cop #1 | Daniel F. Risteen, Jr. |
| Crack House Cop #2 | Francis P. Hughes |
| Crack Addict | Johnny Cicco |
| Exam Instructor | Chance Kelly |
| Bartender at Brasserie | Shay Duffin |
| Bookie in Costello's Bar | John McConnell |
| State Trooper #1 | Kenneth Stoddard |
| State Trooper #2 | Jeffrey Winter |
| New Guy | Paddy Curran |
| New Guy | Mark Philip Patrick |
| Executed Man | David Fischer |
| Waiter in Alley | Walter Y.F. Wong |
| Waiter in Alley | Tony M. Yee |
| Little Girl at Airport | Francesca Scorsese |
| Young Man at Airport | Alex Morris |
| Woman with Dog | Emma Tillinger Koskoff |
| Crack House Denizen | Craig 'Radio Man' Castaldo |
| Triad (uncredited) | Takumi Bando |
| Reporter (uncredited) | Eric Bruno Borgman |
| Irish Thug (uncredited) | Brendan Burke |
| Gang Member (uncredited) | Chris Chinn |
| Crime Scene Unit (uncredited) | Jeffrey Corazzini |
| Cadet (uncredited) | Jim Ford |
| Woman Office Worker (uncredited) | Trudi Goodman |
| Academy Pistol Coach (uncredited) | Steve Lord |
| EMT (uncredited) | Jack McCullough |
| Reporter (uncredited) | Osmani Rodriguez |
| Academy Firearms Instructor (uncredited) | Lawrence Cameron Steele |
| Rugby Fan (uncredited) | Conor Timmis |
| Officer (uncredited) | Joseph Oliveira |
| Southie Teen / Soccer Player (uncredited) | Adam Masnyk |
| Pedestrian (uncredited) | John Franchi |
| Subway Passenger (uncredited) | Paul McGillicuddy |
| MA State Police Detective | Kim Carrell |
| Adult Film Actress (uncredited) | Rebecca Love |
