As a child, Dietz appeared in commercials with her twin sister Marianne DeFossey, and beginning at the age of 12 she started studying acting at the
Neighborhood Playhouse. She made her television debut in 1963 in a small guest role on
The Doctors. Shortly thereafter she landed a recurring role on the
soap opera Love of Life. She made her film debut starring in the 1966 movie
Teenage Gang Debs as Ellie. The following year she portrayed Penny Wohl in the critically acclaimed
independent film ''
David Holzman's Diary''. The film never got much in the way of theatrical distribution despite having Dietz's nude scene featured in Life Magazine's photo spread and in the book of the film. She did not recall if she auditioned for the role of Penny but she added, "it was a fun shoot." Dietz spent much of the late 1960s and early 1970s appearing in theatre productions. She notably appeared
Off-Broadway as the Young Girl in the premiere of
Bruce Jay Friedman's
Steambath at the Truck and Warehouse Theater in 1970. In 1972, she portrayed an androgynous runaway in the premiere of
Joyce Carol Oates'
Ontological Proof of My Existence. Her portrayal in the play led to an invitation to do a screen test for
The Exorcist. She was cast in two memorable roles in the film:
Pazuzu the demon (better known as the face of death), and the 'possessed
Regan' (the
Linda Blair character) in some scenes. She performed in scenes that were too violent or disturbing for Blair to perform, including the crucifix scene and the fistfight with Father Karras. She recalled that director
William Friedkin gave her no notes and said, "I wasn't playing a little girl, I was playing the demon that possessed a little girl." Blair, who recalls Friedkin telling her the film would not succeed if she was not in as many shots as possible, estimates that Dietz is in 17 seconds of the film. Dietz, angry that her contribution to the film had been minimized, claimed in the media to have performed
all the possession scenes. Warners ultimately measured her screen presence at 28.25 seconds, but denied that her contribution was dramatically significant. After
The Exorcist, Dietz had a highly active career on television during the 1970s, appearing as a guest star on such shows as
Planet of the Apes,
Korg: 70,000 B.C.,
Barnaby Jones, and
Happy Days among others. She also portrayed the recurring role of Linette Waterman in the soap opera
The Guiding Light and appeared in the films
You Light Up My Life (1977) and
Parts: The Clonus Horror (1979). In 1980, Dietz joined the cast of
General Hospital as Sarah Abbott, a role she played for several years. She also appeared as a guest star on
Trapper John, M.D. (1982) and in the horror film
Freeway Maniac (1989). More recent film credits include
Naked in the Cold Sun (1997),
Hurricane Festival (1997),
Bad Guys (2000),
Exorcism (2003),
The Mojo Cafe (2004),
Neighborhood Watch (2005),
Constantine (2005),
Karla (2006),
Creepshow III (2006), ''
Dog Lover's Symphony (2006), and Tracing Cowboys'' (2008). Dietz appeared as Winnie Gilmore in
Halloween II (2009) in an uncredited role. Dietz was cast in the thriller
Eden Falls, co-written by
Victor Miller, creator of
Friday the 13th, and then slated for a 2017 release. However, , the project was still in development. ==Filmography==