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Kessler began his career as an actor. His first professional appearance was in the Philadelphia premiere of the play Waiting for Godot, appearing opposite Bruce Dern. He has subsequently appeared in several films, including James Dean (portraying Lee Strasberg, under whom he had studied at the Actors Studio).
Writer and director 
Kessler studied acting with Lee Strasberg and was accepted into the Director's Unit of the Actors Studio, where he wrote and directed his first one act play, The Viewing, which he subsequently directed at the Lucille Lortel Theater in New York.
Orphans (1983) was directed by Gary Sinise and opened at Chicago's Steppenwolf Theatre before starting its successful Off Broadway run. Albert Finney won the Olivier Award for his performance in the London West End production of Orphans.
Kessler's other plays include:
- Robbers (1987) which has been performed at the Long Wharf Theatre in New Haven, Connecticut starring Judd Hirsch
- Possession (1976) which was premiered by the Ensemble Studio Theatre, directed by James Hammerstein.
- Unlisted which was performed at the Tiffany Theatre in Los Angeles starring Charles Robinson.
- Burning Bright
- The Family Circle
- The Engagement
- The Viewing
He has also written:
- The Saint of Fort Washington, for which he was executive producer.
- Touched, in which he also acted.
Other activities 
Kessler and his wife, actress Margaret Ladd, are founders of the Imagination Workshop located at UCLA's Neuropsychiatric Institute. This workshop brings together actors, writers and directors who create scenes and original plays to be performed by psychiatric patients, veterans and "at risk" students in the L.A Public Schools. In 1998 they won the Ovation Award "for their excellence in using theatre to impact and involve a community."
Kessler and Mark Rydell co-moderate the Writer/Directors Unit of the Actors Studio West at Greenway Court Theatre. Kessler served as the Director of the Sundance Screenwriter's Lab. He also participated in the Sundance Screenwriters Conference in Hungary, working with writers from all over Eastern Europe.
Professional seminar 
In 2009, Kessler, working with actor Martin Landau and director Mark Rydell, have teamed up to produce an education seminar, the Total Picture Seminar, a two-day event covering the disciplines of acting, directing and writing for film.