As a theatrical director, Gregg directed the New York and Los Angeles premieres of Kevin Heelan's "Distant Fires" in 1991 and 1993 respectively, as well as the 1996 Atlantic Theater Company revival of Mamet's "Edmond", among other plays. He also won praise for his off-Broadway performance as an abusive nightclub owner in Jez Butterworth's "Mojo" (1997). Meanwhile, a screenplay for a romantic thriller he wrote found no takers, but an interested DreamWorks executive, Nina Jacobson, pitched Gregg a thriller hatched by studio boss Steven Spielberg, and he was hired to write the first draft. When newly-signed director Robert Zemeckis expressed interest in a supernatural thriller, there happened to be one on hand, and Zemeckis, rather than bringing another writer aboard, retained Gregg. The first-time scribe received sole screenwriting credit for "What Lies Beneath" (2000). Starring Harrison Ford and Michelle Pfeiffer, the popular thriller took in a spirited $30 million its first weekend.

Gregg’s ongoing collaboration with Mamet continued in 2000 with "State and Main”, the playwright’s Hollywood satire set in small town Vermont, and the off-Broadway revival of "Sexual Perversity in Chicago”, also by Mamet. After a thankless role as a Cybertronics Supernerd in Spielberg’s doomed “A.I. Artificial Intelligence” (2001), Gregg appeared as Captain Tom Metsker in the well-received Vietnam drama “We Were Soldiers” (2002), starring Mel Gibson as the commander of troops led into the Ia Drang Valley, one of the first—and perhaps worst—major battles in the war. In “Lovely & Amazing” (2002), the critically acclaimed ensemble comedy with Catherine Keener and Jake Gyllenhaal, Gregg played the husband of Michelle (Keener), a self-centered and struggling artist dealing with an unhappy marriage. Clark also appeared that year in Robin Williams’ stab at independent film, “One Hour Photo”, a competent, low-key thriller that took audiences and critics by surprise.

Though Gregg’s main interests were stage and the silver screen, he did pop up on television every now and then. Episodes of “The Shield” (FX, 2001- ), “The West Wing” (NBC, 1999- ), “Sex & the City” (HBO, 1998-2004), and “Will & Grace” (NBC, 1998- ) dotted his resume. Then as chief news executive of CNN, Eason Jordan, Gregg oversaw the cable station’s coverage of the 1991 Gulf War in “Live from Baghdad” (HBO, 2002), the award-winning made-for-TV movie starring Michael Keaton and Helena Bonham Carter. Gregg segued back into film and rejoined writer-director Mamet for the solid, but unnoticed political thriller, “Spartan” (2003). After a minor role in the improbable drama, “The Human Stain” (2004), Gregg surfaced as a sycophantic corporate shark in the lauded comedy-drama “In Good Company” (2004), starring Dennis Quaid, Topher Grace and the ubiquitous Scarlett Johansson.