 Both series are usually in continuous production 52 weeks a year, so in order to produce the first crossover, work on both shows has to be halted for two weeks to release a number of cast members to appear in the special.Once regular production began again, the availability of cast members set to appear in the crossover was limited, and both series had to rely for the most part on the remaining characters who were not in the special.Filming took six weeks to complete, and occurred for the most part at "Casualty" emergency department in Bristol.