Drawn to theatre as a means of reinventing herself, Valk entered New York University's drama department at 19 and spent two years studying with world-renowned acting teacher Stella Adler before encountering Elizabeth LeCompte, The Wooster Group's director, and the troupe's other co-founders at NYU's then-new Experimental Theatre Wing. In 1979, she joined the collaborative ensemble, first making costumes, then as a stage manager, and, finally, in 1981, performing her first role in Route 1 & 9, a controversial version of Thornton Wilder's Our Town in which the Group recreated black vaudeville routines. She has acted in every Wooster Group play ever since. “I have so much power and liberty working with the Wooster Group at the Performing Garage,” she says.
Valk's lifetime of unorthodox portrayals include pivotal performances in L.S.D. (…Just the High Points…) (1984), based on Arthur Miller's The Crucible, and Brace Up!, from Chekhov's Three Sisters (1991). More recently, she has appeared in such deconstructed classics as the Gertrude Stein-inspired House/Lights (1997/2005); Eugene O'Neill's The Emperor Jones (1993/2007), in which, in blackface and a Kabuki-influenced costume, she transgressed the boundaries of race and gender; and an avant-garde Hamlet (2006/2007), playing Gertrude and Ophelia.
Although she has also appeared in films by directors Jonathan Demme, Raúl Ruiz and Peter Sellars, Valk is a consummate stage actress who has received numerous awards for her work, among them an OBIE for Sustained Excellence in Performance (1998) and a Bessie Award for Best Performer (2002).