In addition to writing several segments of Inspector Morse (1987), he also appeared (as an uncredited extra) in most of these stories, as did the series' original creator, Colin Dexter.
[on John Thaw] John somehow manages to exclude himself from the performance. He has this wonderful quality of stillness to him but he still manages to get across what he's thinking. I once asked him how he did it and he simply said imagination. He imagines himself into a role with such vividness that he's able to convey it.