A rehearsal is a preparatory event in music and theatre (and in other contexts) that is performed before the official public performance, as a form of practice, and to ensure that all details of the performance are adequate for professional presentation.
The gathering of actors and director to put a play into production; the period in which the director stages the play and the actors develop and repeat their dialogue and actions; etymologically, a "reharrowing," or repeated digging into. In French, the comparable term is répétition.
In television, rehearsal involves the director, a dialogue coach and actors who will come ready to block the action and define their character arc ñ or character journey, as they look for meaning in their lines. Theatre actors will look for the intention or objectives of the character in the scene and ask themselves what the character is trying to achieve at any given moment. They learn their lines perfectly outside of rehearsal.
The Superstition: Professional actors consider it a bad sign if a rehearsal is perfect. The play will have a very short run after a perfect rehearsal, or will go very badly. Similarly, it is extremely unlucky to speak the tag line, or the last line of the play, during rehearsals. The line which completes the play must not be spoken until the opening night of the show. The Origin: After a perfect rehearsal a cast and crew tends to feel as if they are 'prepared' for the production. They lose their nervous edge and adrenaline and, believing themselves to be fully prepared, stop paying close attention while on stage. The last line of a show completes the play, and a production is never complete until it is before an audience.