Gilliam’s film career began in the early 1990s. He has starred in award-winning films such as Still Alice. His other film credits include Interview with a Vampire in 1994, Sergeant Sugar Watkins in the 1997 action film Starship Troopers and Sergeant Steven Altameyer in the 1996 film Courage Under Fire.
Gilliam has had a number of recurring roles on television, including during the seventh season of The Cosby Show as Aaron Dexter, boyfriend of Erika Alexander’s character, and as Alan Deaton on Teen Wolf. His most prominent role has been on The Wire. In 2008, he attracted attention for criticizing the show being overlooked for Emmy nominations despite its critical success. He has had a number of guest roles in other shows such as CSI: Miami, Nurse Jackie, Skins, Person of Interest, and The Good Wife.
He played Clayton Hughes on Oz. In May 2014, Gilliam was cast as Father Gabriel Stokes in the fifth season of AMC’s The Walking Dead. Along with Chad Coleman and Lawrence Gilliard Jr., he is the third The Wire alum to join the series.
Gilliam has acted on stage, playing Prince Edward in a 1993 production of Richard III. He played the title role in Othello, first in 2010 at Boston’s Commonwealth Shakespeare Company and then the following year at the Houston Shakespeare Festival, directed by his wife, Leah C. Gardiner. The pair returned to Houston in 2013 for Antony and Cleopatra, with Gilliam playing Antony and Gardiner directing.