As a general rule of thumb, film sequels rarely live up to the original. Although there are some exceptions, writers and directors generally fail to reproduce the quality and intensity of the original idea.
The same holds true for television series. Many of the best TV series have been stopped at their peak. No Angels recently bowed out at the end of its third run. In stark contrast, Teachers went off the rails as it was stretched out over four series. Red Dwarf lost much of its sparkle as the crew meandered through the galaxy for a total of eight series.
Yet there are exceptions which are reinvigorated each season, and are long lasting.
Spooks is a good example. Seemingly with a named bullet for each member of the cast, this sometimes harsh recycling has kept Spooks fresh for four seasons.
The fifth run of ten one-hour programmes begins this autumn, and the commissioning of a sixth series has just been announced.