He then wrote the screenplay for the 2001 film adaptation of Tape, directed by Richard Linklater and starring Ethan Hawke, Uma Thurman, and Robert Sean Leonard.
In 2008 his play Fault Lines was produced by Naked Angels at The Cherry Lane Theater in New York, directed by David Schwimmer and starring Josh Lucas, Noah Emmerich, Jennifer Mudge and Dominic Fumusa.
Also in 2008, his play Geometry of Fire was produced by Rattlestick Theater in New York, starring Jennifer Mudge, Jeffrey Demunn and Kevin O’Donnell, and directed by Lucie Tiberghien.
In 2010, Belber's play Dusk Rings A Bell was produced by New York's Atlantic Theater Company, starring Kate Walsh and Paul Sparks, and directed by Sam Gold.
In 2012, Belber's play Don’t Go Gentle was produced by New York's Manhattan Class Company, starring Michael Cristofer, Jennifer Mudge, Maxx Brawer, David Wilson Barnes and Angela Lewis, and directed by Lucie Tiberghien.
Also in 2012, his play The Power Of Duff received the first of 3 domestic productions, at New York Stage & Film, the first of which starred Greg Kinnear, Jennifer Westfeldt and Dominic Fumusa.
Belber's other plays include The Transparency of Val, The Wake, Through Fred, The Death of Frank, Mel and Gene, Drifting Elegant, and Finally.
In film he has sold spec scripts to Sony, Universal and Paramount, and done numerous polishes and rewrites (including uncredited work on Dallas Buyer’s Club).
Belber created the 2024 Netflix television series, The Madness, starring Colman Domingo, Marsha Stephanie Blake, and John Ortiz.