 By 2005 , Nesbitt had become tired of the formula and threatened to quit unless the structure of the series was changed . He was made a creative consultant and suggested that Murphy keep one undercover role for a full series , instead of changing into a new guise every episode . This new dramatic element to the series was intended to make it a closer representation of real @-@ life undercover work . Alongside his research with former undercover officer Peter <unk> , Nesbitt hired a personal trainer and grew a <unk> moustache to change Murphy 's physical characteristics and tone down the " <unk> <unk> " persona that the audience had become accustomed to from his roles . With his trainer , he worked out three times a week , boxing and doing circuits and weights . After the first new episode was broadcast , Sarah <unk> wrote in The Times , " In the past , when attempting a nasty stare or a hard face , Nesbitt has never managed much more than a <unk> <unk> look , <unk> forever threatening to break out behind those <unk> Irish eyes . But here , it 's different . He genuinely has the air of a man who means business . " The refreshed series marked another milestone in Nesbitt 's career ; he describes it as " a big moment " in his life . Murphy 's Law was not recommissioned for a sixth series , which Nesbitt attributed to the damage done to the fifth series ratings when it was scheduled opposite the popular ITV drama Doc Martin . 
