When the network approves a pilot for filming, several things must occur. First, the showrunner and producers are hired. The showrunner is the person in charge. He or she works with the writers and script, casts the actors, is responsible for creative direction and usually oversees the project from start to finish. Sometimes the showrunner is the person who created the show's concept, wrote the script or treatment and pitched it.
Producers help the showrunner handle everything. They help with hiring the director, talent, crew, writers and assistant producers, and their first task is rewriting or updating the script. When that's done, cast auditions are next. Then the producers hire the crew -- and finally, the pilot is shot and edited. This schedule is generally outlined as preproduction, production and postproduction.