The Superman cast members had restrictive contracts which prevented them from taking other work that might interfere with the series. Except for the second season, the Superman schedule was brief (13 shows shot two per week, a total of seven weeks out of a year), but all had a "30-day clause", which meant that the producers could demand their exclusive services for a new season on four weeks' notice. This prevented long-term work on major films with long schedules, stage plays which might lead to a lengthy run, or any other series work.