Noted for his work in avant-garde theater, Diehl has performed in more than 140 films and television shows, including Land of Plenty, Stripes, City Limits, Nixon, Jurassic Park III, and the TV series Miami Vice, The Shield, and Point Pleasant.
[7] Diehl moved into a basement apartment in Greenwich Village in New York, and despite a drastic reduction in his income, he declined television roles, and chose instead to continue working in film while pursuing a serious stage career.
[7][10][11] In 1997, Diehl reprised his role in Action at the Public Theater, and in 2005 he worked once again with Padua Playwrights, appearing in two plays in Mednick's Gary Trilogy.
Among others, his theater credits include Life of Mine, (with Holly Hunter) at the Mark Taper Forum, Samuel Beckett's Endgame and Happy Days, one of three plays which Diehl directed.
[12] He has appeared in more than 140 films, including Joysticks (1983), Angel (1984), City Limits (1984), Madhouse (1990), The Dark Side of the Moon (1990), Kickboxer 2 (1991), Mikey (1992), Mo' Money (1992), Gettysburg (1993), The Client (1994), Stargate (1994), The New Age (1994), Mind Ripper (1995), A Time to Kill (1996), Pearl Harbor (2001), and Road to Nowhere (2010).
[13] He played G. Gordon Liddy in Oliver Stone's Nixon (1995), the mercenary Cooper in Jurassic Park III (2001), and in recurring roles on The Shield, Friday Night Lights, The West Wing and The John Larroquette Show.
Shot on digital video in sixteen days, the film centered on Diehl's character, Paul, a troubled Vietnam veteran, and his niece, played by Michelle Williams.
Scott wrote that Diehl gave a "wry, cunning performance, allowing glimmers of Paul's intelligence and decency to shine through even in his moments of high self-delusion.