A stage filled with dancers must be a controlled area, and each person needs to know exactly where they should be at all times. Large dance numbers do not spontaneously occur, and they are directed by a choreographer. This is the person who chooses the style of movement to accompany the music, and they break each musical number into a set routine for the dancers who will be on stage.
The work of the choreographer begins with learning the selection of the music, and they then decide how many dancers will perform the piece. Basic movements are chosen as part of the style of the dance, and the choreographer plans where each dancer will stand as the music begins. Step by step, each move is mapped out before a performer is ever chosen. Once a group of dancers have been accepted, it is the job of the choreographer to teach them the moves they will make and rehearse with them until the dance is perfect.