In opera, a répétiteur is the person responsible for coaching singers and playing the piano for music and production rehearsals.
In addition to being able to sight read piano parts, some répétiteurs can play on the piano an orchestral score reducing it in real-time (orchestral reduction), by reading from a large open score of all of the instruments and voice parts.
Répétiteurs are also skilled in following the directions of a conductor, in terms of changing the tempo, pausing, or adding other nuances.
[2] In ballet, a répétiteur teaches the steps and interpretation of the roles to some or all of the company performing a dance.
[1][3] Several late 20th-century choreographers, such as George Balanchine, Jerome Robbins, Gerald Arpino and Twyla Tharp, have established trusts and appointed conservators—hand-picked dancers who have intimate knowledge of particular ballets—as répétiteurs of their works.