While in college Dunstan experimented with 16mm film often using himself as the guinea pig in poorly executed Stunts that left him dragging behind a car, the target of flying knives, and thrice being set on fire. He holds the record for spilling the most fake blood ever in a student film. Since moving to Los Angeles in 1999, Dunstan toiled in several odd jobs including a retirement home, an Optometry clinic, and a video store while spending his nights writing on a borrowed computer from fellow Screenwriter, Patrick Melton.

In March 2005, Dunstan and Melton won Season Three of Project Greenlight with their screenplay, Feast, which was directed by John Gulager. The film starred Krista Allen and Eric Dane, and was produced by Dimension Films and Neo Art & Logic.

In addition to writing and directing, he and writing partner Patrick Melton have made cameo appearances in Saw V, Supergator, Feast and The Collector, appearing in the later two through the same puppet show clip shown on TV screens. He, Melton and Stephen Romano co-wrote the thriller novel Black Light - Don't Look Too Deep, which will published on October 5, 2011 over Mulholland Books as hardcover. The novel will adapt in a feature film written by Patrick Melton and Dunston, under the direction of Stephen Romano, Michael De Luca will produce for Warner Bros. Dunstan co-wrote with Patrick Melton a screenplay for potential eleventh film of the Halloween Franchise.

Dunstan has written the screenplays for Feast, Feast 2: Sloppy Seconds, Feast III: The Happy Finish, The Collector, Saw IV, Saw V, Saw VI, and Saw 3D. He is also in pre-production and post-production for several of his other screenplays. In addition to writing the screenplay for The Collector, he directed the film. He also directed the sequel to The Collector, titled The Collection. The film, produced by Liddell Entertainment and Fortress Features, was released in 2012.

In 2013, Melton and Dunstan were enlisted to perform uncredited rewrites on Pacific Rim when their spec script Monstropolis caught Guillermo Del Toro's attention, and then were attached to a film version of God Of War. On October 2016, Dunsten, along with partner Melton, were selected to write for the adaptation of Brandon Sanderson's The Way of Kings. He also wrote additional dialogue for the Saw Escape Room in Las Vegas.