As a British TV watcher, my opinion is that the best British drama and comedy programmes have an optimum of 12 episodes or thereabouts. Anything more tends to dilute the quality. For example, John Cleese deliberately ended Fawlty Towers after 12 episodes because he knew that he could not sustain the level for any more episodes. Similarly, Jonathan Lynn and Anthony Jay stopped writing Yes, Minister and Yes, Prime Minister for the same reason - they felt that they were in danger of exhausting the same line of humour. Our drama series too tend to be shorter (e.g. Survivors which is now finishing its second (and probably final) series. Yet US shows seem to take on a life of their own, often to the detriment of the overall quality. I remember watching a series of Lost, one of Heroes and also 24 before giving up, because the novelty paled and each of these ventures appeared to me to be becoming narcissistic and even existential - they caried on merely to be there, not to improve. Even excellent quality comedy such as Cheers and Frasier seemed to me to suffer from the repetition of the same running jokes. After several years of Frasier[b][/b], I could honestly say that I could not have cared less whether Daphne and Niles ended up together, and the constant joke became an irritation. Yet British friends of mine disagree, preferring the American style of creating something long-running, and suggesting that this leads to greater depth of character and greater audience empathy with the protagonists.
I am not suggesting that British writing is better than American writing - good writing is to be found on both sides of the Atlantic. What I am pondering is why US series tend to spawn multiple and long-running series (seasons). Is it a genuine desire to create a much broader canvas for a story or a comedy? Or, is it (to be cynical) an opportunity for professionals (writers, producers, directors and distributors) to milk a cash cow, even if this means diluting the purity of the milk (as Alan Partridge might have said)?
