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William Reibert Mapother Jr. (; born April 17, 1965) is an American actor, known for his role as Ethan Rom on the television series Lost and starring in the film In the Bedroom. He is also known for the film Another Earth.
| Year | Title | Role | Notes |
| 1989 | Born on the Fourth of July | Platoon Member – Vietnam | |
| 1998 | Without Limits | Bob Peters | |
| Trickle | Josh Riddell | ||
| The Unknown Cyclist | By-Stander | ||
| 1999 | Magnolia | WDKK Show Director's Assistant | |
| 2000 | Mission: Impossible 2 | Wallis | |
| Almost Famous | Bartender | Scenes deleted | |
| 2001 | In the Bedroom | Richard Strout | |
| Swordfish | Gabriel's Crew | ||
| 2002 | Minority Report | Hotel Clerk | |
| Self Storage | Graham | ||
| 2003 | The Kiss | Peter | |
| 2004 | Suspect Zero | Bill Grieves | |
| The Grudge | Matthew Williams | ||
| 2005 | Lords of Dogtown | Donnie | |
| The Zodiac | Dale Coverling | ||
| Chloe | Doctor | ||
| 2006 | Ask The Dust | Bill | |
| The Lather Effect | Jack | ||
| World Trade Center | Jason Thomas | ||
| 2007 | Moola | Bob | |
| Moving McAllister | Bob | ||
| 2008 | The Burrowers | Will Parcher | |
| Hurt | Darryl Coltrane | ||
| 2010 | A Warrior's Heart | David Milligan | |
| 2011 | Another Earth | John Burroughs | |
| Citizen Gangster | Detective Rhys | ||
| 2012 | FDR: American Badass! | Dr. Ellington | |
| 2013 | Underdogs | Bill Burkett | |
| 2014 | I Origins | Darryl | |
| 2015 | The Atticus Institute | Dr. Henry West | |
| Blackhat | Rich Donahue | ||
| 2016 | Tell Me How I Die | Dr. Jerrems | |
| 2024 | Outlaw Posse | Angel |
| Year | Title | Role | Notes |
| 2002 | CSI: Crime Scene Investigation | Douglas Sampson | Episode: "Snuff" |
| The Pennsylvania Miners' Story | John 'Flathead' Phillippi | TV movie | |
| Touched by an Angel | Eddie | 2 episodes | |
| 2003 | Law & Order: Special Victims Unit | Luke Edmunds | Episode: "Rotten" |
| 2004 | Line of Fire | Larry | 2 episodes |
| CSI: Miami | Pete Keller | Episode: 'Lost Son" | |
| NCIS | Kyle Grayson | Episode: "See No Evil" | |
| Crossing Jordan | Henry Bishop | Episode: "Death Goes On" | |
| 2004–2010 | Lost | Ethan Rom | 11 episodes |
| 2005 | The Inside | Ronald Ewing | Episode: "Skin and Bone" |
| Threshold | Gunneson | 2 episodes | |
| 2006 | Robot Chicken | Alien Bully / Leprechaun | 2 episodes |
| 2007 | K-Ville | Gordon Wix | Episode: "Pilot" |
| Viva Laughlin | Sweet Lenny Collins | Episode: "What a Whale Wants" | |
| WordGirl | Guy Rich (voice) | Episode: "The Young and the Meatless/Mr. Big's Colossal Mini-Golf" | |
| 2008 | Skip Tracer | Henry Hargrove Jr. | TV movie |
| Lost: Missing Pieces | Ethan Rom | Episode: "Jack, Meet Ethan. Ethan? Jack." | |
| Criminal Minds | Ian Corbin | Episode: "Paradise" | |
| 2009 | Prison Break | FBI Agent Chris Franco | 2 episodes |
| 2010 | Human Target | Sam Fisher | Episode "Sanctuary" |
| I Heart Vampires | William | 2 episodes | |
| Law & Order: Criminal Intent | John Silvestri | Episode: "Love on Ice" | |
| 2011–2014 | The Mentalist | Richard Haibach | 4 episodes |
| 2012 | Burn Notice | Garret Hartley | Episode: "Shock Wave" |
| Justified | Delroy Baker | 2 episodes | |
| American Horror Story | Misogynist Driver | Episode: "Dark Cousin" | |
| 2013 | Mad Men | Randall Walsh | Episode: "The Flood" |
| Castle | Carl Matthews | Episode: "The Disciple" | |
| 2014 | Hawaii Five-0 | Eric Porter | Episode: "Ka Makuakaneka" |
| 2015 | Constantine | Jacob Shaw | Episode: "A Whole World Out There" |
| Grimm | Dwight Eleazar | Episode: "The Believer" | |
| 2016 | Supergirl | Rudy Jones / The Parasite | Episode: "Changing" |
| 2017 | MacGyver | Daniel Horn | Episode: "Hole Puncher" |
| 2018 | The Blacklist | Bill | Episode: "Ruin" |
| 2020 | 9-1-1: Lone Star | Bruce Ackerman | Episode: "Monster Inside" |
| 2021 | FBI Most Wanted | Travis Russel | Episode: "Winner" |
| Year | Title | Role | Notes |
| 2010 | Fallout: New Vegas | Colonel James Hsu | |
| 2012 | Hitman: Absolution | Agent 47 | Motion capture |
| 2016 | Hitman | Dino Bosco |