Supernumerary actor

Supernumerary actors are usually amateur character actors in opera and ballet performances who train under professional direction to create a believable scene.
Definition.
The term's original use, from the Latin supernumerarius, meant someone paid to appear on stage in crowd scenes or in the case of opera as non-singing small parts. The word can still be found used for such in theatre and opera. It is the equivalent of "extra" in the motion picture industry. Any established opera company will have a supernumerary core of artists to enhance the opera experience. The Metropolitan Opera (Met) in New York and the Washington National Opera are known for their high profile and seasoned supernumeraries. The WNO saw its supernumerary golden age under the direction of English actress Jennifer Crier Johnston, who was supernumerary director for eight years (1998–2006). Ms. Johnston appeared in classic Hollywood movies such as "My Fair Lady", "The Unsinkable Mollie Brown", "The Americanization of Emily", and "The Sound of Music". "The Washington Times" ran an exhaustive article on supernumeraries in November 2002, in which a Jennifer Johnston describes in detail the fine craft of a supernumerary in the opera.
The WNO has had some major supernumerary personalities on stage such as U.S. Supreme Court Justices Ginsburg and Kennedy who made their last appearance in the opening of Strauss comedic opera "Die Fledermaus". New, promising, and exciting supernumeraries at the Washington National Opera include: Marlene Hall, David Brindley, Michael Walker, Emily Cohen, Eric Schultz, Felipe Lagos, Victor Yager, John Tinpe, Rey Rivera, Samantha Smith, Liam McKenna, and Toni Smiley. Other long time famed supernumeraries include Fernando Varisco, Karl Moeller, Patrizia DiZebba, Harry Spence, Peter Whitten, Alex Riley, Gary Nooger, and Alain Letort. In 2005 Walker, Yager and Varisco were invited by the well known opera director Cindy Oxberry, who has been assistant director for the WNO for over 10 years, to work in a brand new production of The Washington Savoyards' "The Mikado". Oxberry's style and force were obvious in these performances and the "Washington Post" agreed that "The Savoyards fully captured the energy of Arthur Sullivan's inimitable melodiousness and the thrust of William Schwenck Gilbert's satiric dialogue, riddled -- perhaps a little too obviously -- with updated political jabs."
Typical supernumerary work.
Supernumeraries are usually amateur character artists who train under professional direction to create a believable scene. They almost become part of the props and give a sense of credibility to scenes where crowds, court assistants, lackeys, peasants or a variety of period characters are needed. Ms. Johnston's style emerged through her experiences in Hollywood and the British theater and she coached all of her supernumeraries until the character, the movements, pose and demeanor matched the period of the opera being presented. Rehearsals can last from two weeks to several months depending on the complexity of the performances. Some operas require over 50 supernumeraries. Work is assigned according to the ability to look the part and in many cases by the costume size since many of the productions are borrowed from other major opera houses. The Varisco-Johnston style of "supering" emphasizes an understated performance that doesn't "steal focus from the main actors" but it is still vibrant and effusive. Other styles have evolved like the method acting of Yager and Walker. 
The repertory at any established opera house certainly includes operas with many supernumeraries. Setting a record, with 227 supers, was a new production of Prokofiev's "War and Peace", which had its last performance of the season on March 19, 2006. Other operas at the Met and other great opera houses awash in supers include "Aida" (the triumphal march with 165) and Puccini's "La Bohème" (the Café Momus scene with 125), according to François Giuliani, press director of the Met. (Those numbers do not include soloists, members of the chorus and dancers.) But the chance of being a super is pretty much limited to those with experience and "people who can take direction," said Bob Diamond, administrator of supernumeraries at the Met. "We don't take people off the street," he added. It is commonly accepted that an opera will use ten men as extras for every woman. "That's opera, and we have no control," Mr. Diamond said. The commitment in terms of time can range from as little as one two-hour rehearsal and a dress rehearsal—but supers do have to agree to take part in a minimum number of performances—to as much as five hours a day for four or five weeks for, say, a newly staged opera. 
Supernumerary work keeps evolving as operas evolve. There has been a merger of techniques used in Broadway, movies and opera. A good illustration of this type of merging is seen for the second time at the WNO 2006 Fall opera season with the presentation of Puccini's "Madama Butterfly", wherein Polish director Mariusz Treliński, originally a movie director, presented his innovative production which used an extensive cast of supernumeraries as Japanese fishermen, Yamadories and live statues. The "Washington Blade" took notice of these statues and described them effectively: 
"A wall of Japanese statues come to life and drop flower petals from the heights of the opera stage at Pinkerton’s impending return..."
Having said this, directors sometimes take too many artistic liberties with some operas and the final result is not what is to be expected. This can be illustrated with Marta Domingo's production of "La Traviata" presented at the Washington National Opera in May 2004. Supernumerary Varisco played the part of death during the last act, and this is what the "Washington Post" said about this supernumerary character:
Stage director Marta ("Mrs.") Domingo seems to have taken the Hippocratic oath as her inspiration for this "Traviata": She does no harm. Her conception is pretty generic, though—ballrooms and sickbeds, bright lights and heavy curtains—and the few directorial "touches" I could discern (the shadow of death marching by with a scythe outside Violetta's window) were not especially effective. Imagine a production, any production, of "La Traviata" and you'll probably have a pretty good idea of this one.
Supernumeraries can add a very dramatic effect when they are properly used in grand opera productions such as "Aida" and "Le Cid". The logistics of managing such a corps require a supernumerary coordinator with experience and talent. In the case of "Le Cid", supernumeraries had to be trained for six weeks by a professional stunt expert in the art of medieval fighting brought from Spain specifically for this purpose. Opera critics can note the effect of a sizeable supernumerary corps:
The stage layout is sensible but compelling--a set of ascending stairs running deep upstage, sometimes topped by a very impressive squadron of cavalry in full armor. Crosses, gates and moving side panels delineate different scenes. De Ana uses his very large chorus, supplemented by athletic supernumeraries, with precision: They move quickly, creating striking tableaux that bristle with bellicose energy.