These 19th-century lines of business were considered a step above supernumerary in that the characters that fell to these actors might occasionally speak lines. In his 1880 memoir The Stage, James E. Murdoch notes that a walking gentleman would play a character who is "essential to the progress and development of the plot," but who has "a merely mechanical part in the dialogue" (204-5). These parts, like Rosencrantz and Guildenstern in Hamlet, were useful for giving young actors the opportunity to observe the techniques of their betters at close range.
The Historical Dictionary of the American Theater. James Fisher.