Although most familiar for his later television and movie roles, Marshall also had a distinguished Broadway career. In 1948, having already appeared in the original New York productions of The Skin of Our Teeth and The Iceman Cometh, Marshall would join Marlon Brando, Montgomery Clift, Julie Harris, Kim Stanley, and 45 others to make up the first group of actors granted membership in the newly formed Actors Studio. In subsequent years, he'd land the leading roles in The Crucible and Waiting for Godot. In 1973, he returned to the live stage to play the title role in a highly praised production of Macbeth in Richmond, Virginia, under the direction of Keith Fowler. From January 1974 until February 1982, Marshall was an occasional participant and the original host of the popular nightly radio drama, The CBS Radio Mystery Theater.