Whenever a project requires a musical number, the director will hire a choreographer to plan the dance steps and train the performers. The choreographer will read the script, and then they will listen to the music that has been written for each number. It is their job to select the type of dance steps that will best fit the entire project, and they will then begin planning the dances from that point. Their work is important because it will be very noticeable, and the musical numbers must be fully integrated into the show.
Choosing types of dance is often an easy project, and many project producers already know what they need. Ballet dancing would generally be out of place in a musical about modern life, but it would fit in if one of the main characters was a ballerina. The choreographer may have the option of several different types of dance genres, but the majority of musicals will require the same genre throughout the performance.
Projects of this type generally have a casting call for actors and dancers. The choreographer will choose the majority of the dancers during these sessions, but the producers and director often choose the main characters. If the person cannot dance, it is the work of the choreographer to teach them every step. They will work with them on a regular basis, and this will help the actor learn the physical part of their performance.
Professional dancers are expected to know a wide range of different dance types and steps, and teaching them part of the routine is often how casting decisions are made. The choreographer will start them off with the first group of steps, and see if they can follow directions precisely. Those who falter or cannot keep pace will be dismissed, and then the choreographer teaches them more steps until casting is complete.