Duffey is best known for his role as Brian, the Chotchkie's waiter in the 1999 comedy Office Space. Among his other projects, he appeared in a recurring role in 2001 in Buffy the Vampire Slayer.
- Across Five Aprils (1990) as Jethro Creighton
- In the Name of Love: A Texas Tragedy (1995) as Davey Coombs
- Carried Away (1996) as Young Joseph
- Office Space (1999) as Brian
- The Black Rose (2000) as Kyle
- Buttleman (2003) as Harold's Boss
- Barney: Let's Go to the Zoo (2003) as Scooter McNutty
- Burning Annie (2004) as Tommy
- The SpongeBob SquarePants Movie (2004) as Concession Guy (voice)
- Slaughterhouse of the Rising Sun (2005) as Robert Lewis
- Hollywood Kills (2006) as Ken Vincent
- The Last Lovecraft: Relic of Cthulhu (2009) as Cult member #1
- Death Inc. (2011) as Plumber Jim
- God's Country (2012) as Adam
- Steel Cherry (2012) as Marvin Steel
- Walker, Texas Ranger (1 episode, 1995) as Paul Kelly Moore
- Barney & Friends (47 episodes, 1997-2000) as Scooter McNutty
- ER (1 episode, 2000) as Mitch
- Buffy the Vampire Slayer (6 episodes, 2001) as Murk
- The Drew Carey Show (1 episode, 2002) as Prom DJ
- George Lopez (1 episode, 2003) as Teenage Boy
- Charmed (1 episode, 2003) as Satyr
- The O'Keefes (1 episode) as Beer Guy
- That '70s Show (1 episode, 2004) as Guy #2
- Tony Hawk's Project 8 (2006) (as Robert Duffey)
In 1991, Duffey received a Western Heritage Award from the National Cowboy Museum for his starring role as Jethro Creighton in the 1990 film Across Five Aprils adapted from the Irene Hunt novel of the same name.
- Sime, Tom (May 18, 2012). "Burlesque nails down acting". Dallas Morning News. Retrieved August 22, 1997. Check date values in:
- Thomas, Kevin, Los Angeles Times (Februal 21, 1999). "Take this Job and Shove it Right up the Corporate". Press of Atlantic City. Retrieved May 18, 2012. Check date values in:
- Singer, Jon (February 13, 2004). "Duffey makes Chotchkie's waiter unforgettable". Lumino Magazine. Retrieved May 18, 2012.
- Michels, Patrick (May 18, 2012). "Ten Years On, Office Space Returns!". Dallas Observer. Retrieved February 9, 2009.
- Kiefer, Jonathan (September 4, 2009). "Faster Filmmaker Q&A: "Extract" Writer-Director Mike Judge..." The Faster Times. Retrieved May 18, 2012.
- Stafford, Nikki (2007). Bite Me!: The Unofficial Guide to Buffy the Vampire Slayer. ECW Press. pp. 272, 276. ISBN 1550228072.