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The most recent trend has been for the networks to reduce the number of episodes produced for returning series from 39 per season in the 1950's to, at best. 22 this season. This week CBS announced some changes to their successful Bob Hearts Abishola series. Effective with coming fifth season, only Billy Gardell [Bob] and Folake Olowofoyeku [Abishola] will return as main cast members and will appear in all currently scheduled 13 episodes. All other cast members have been reduced to recurring status and are guaranteed only five episodes. I would assume recurring status pays significantly less than a main character. I suspect this tactic will spread to all the networks and mostly affect aging, expensive but still immensely popular series likeĀ CBS's FBI/NCIS franchises and NBC's "Chicago" Wednesday and "Law & Order" Thursday.
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Look at the UK where a standard series is 6 or 13 episodes and often doesn't appear every year (a model US cable channels have copied)