8 Reasons Behind the Long-Running Success of 'NCIS'

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• With over 400 episodes under its belt, 'NCIS' has surpassed 'Gunsmoke' as the longest-running live-action American TV series, thanks to its winning formula of relatable characters and compelling crime stories
• The show's casting director, Louis DiGiaimo, has been with 'NCIS' since the beginning, helping to assemble an ensemble cast that has maintained viewer loyalty for two decades, including lead actor Mark Harmon, who has starred in every season