(written from a Production point of view)
Erich Anderson is the actor who played the Satarran impostor "Keiran MacDuff" in the Star Trek: The Next Generation fifth season episode "Conundrum" in 1992. He filmed his scenes for this episode between Monday 18 November 1991 and Wednesday 20 November 1991 and Friday 22 November 1991 and Tuesday 26 November 1991 on Paramount Stage 8 and 9.
Anderson has had a very lengthy career in television, with recurring roles in no less than five popular television series: thirtysomething as Billy Sidel; Felicity (co-created by J.J. Abrams as Dr. Edward Porter; NYPD Blue (starring Gordon Clapp) as Don Kirkendall (including two episodes with Gwynyth Walsh); Boomtown (starring Neal McDonough) as Ben Fisher; and Close to Home (starring John Carroll Lynch) as Detective George Branch. He also made guest appearances on Murder, She Wrote (with William Windom), Dallas (co-starring John Anderson, Glenn Corbett, Leigh J. McCloskey, Derek McGrath, William Smithers, Leigh Taylor-Young, and Morgan Woodward), Melrose Place, Sisters (with Stephen Collins), 7th Heaven (also with Stephen Collins, as well as Catherine Hicks), Touched by an Angel, Chicago Hope (in an episode with Susanna Thompson), ER, The X-Files, Family Law (starring Christopher McDonald), CSI: Miami, and Medical Investigation (starring Neal McDonough), among many other shows.
After his appearance on Star Trek, Anderson appeared with TNG co-star Brent Spiner in two more projects: a 1995 episode of The Outer Limits and the 2004 television movie Jack. Other regular Trek performers whom Anderson appeared with outside of Trek are Scott Bakula (in a 1989 episode of Quantum Leap, along with Dean Stockwell), Jolene Blalock (in a 2000 episode of CSI in which she played his wife), Denise Crosby (in a 1991 episode of Sisters, along with Frank Kopyc), Jeri Ryan (in a 2001 episode of Boston Public), Armin Shimerman (in a 2002 episode of For the People), and Connor Trinneer (in a 2006 episode of Close to Home directed by Roxann Dawson).
Prior to any of these, Anderson and his TNG co-star Jonathan Frakes were among the many Star Trek alumni who appeared in the epic 1986 Western mini-series Dream West. The other Trek performers who appeared in this series were F. Murray Abraham, Jeff Allin, John Anderson, Lee Bergere, James Cromwell, Michael Ensign, Alice Krige, Matt McCoy, Glenn Morshower, Fritz Weaver, Noble Willingham, and Anthony Zerbe. He made his feature film debut in the 1984 horror movie Friday the 13th: The Final Chapter (co-starring Lawrence Monoson) followed with a role in the popular actioner Missing in Action (with Lenore Kasdorf) that same year. Later film credits include Patty Hearst (1988, with Kitty Swink), Bat*21 (1988, with Clayton Rohner), The Glass Shield (1994, with Bernie Casey, Victoria Dillard, Jim Fitzpatrick, Natalia Nogulich, Lori Petty, and Biff Yeager), Nightwatch (1997, with Brad Dourif), Where's Marlowe (1998, with Miguel Ferrer and Clayton Rohner), Beyond Suspicion (2000, with Casey Biggs and Jack Kehler), and Unfaithful (2002). Anderson was also involved in a number of TV movies, including 1991's Love Kills starring Virginia Madsen and Jim Metzler, 1992's Overkill with Marc Alaimo and Jack Shearer, 2002's Due East with Clara Bryant, and 2003's A Time to Remember, co-starring Louise Fletcher, Rosemary Forsyth, and Megan Gallagher.