During filming, Iain Cuthbertson, who played Desmond McNutt, was recovering from a heart attack and a stroke which had left him unable to remember his lines, so the crew (and on occasions the other actors in scenes with him) had to hold his lines on large boards so he could read them.
This episode is loaded with masonic symbolism. In particular, look out for masonic handshakes, the tie-pin and pointed handkerchief combo (symbolising the Masonic square and compasses), the three pillars in certain room, Masonic turns of phrase... and most obviously, at the Magic Flute performance the cast appearing in quasi-Masonic robes.
Masonic penetration of the British police has long been a controversial subject. While many police are Freemasons, some have questioned the role of this fraternity in law keeping. Supporters claim it is just incidental, and detractors claim it interferes with procedure.
Although at no point is it suggested that Lewis is a Mason, the term "Lewis" has special significance in Freemasonry. A "lewis" is the son of a Freemason, who is to be initiated at the age of eighteen, rather than the usual twenty one. In operative masonry, it signifies an iron cramp, which is inserted in a cavity prepared for the purpose in a large stone, so as to give attachment to a pulley and hook, whereby the stone may be conveniently raised to any height, and deposited in its proper position.
Colin Dexter, Julian Mitchell: during the opening scene of the Magic Flute rehearsal, as two members of the chorus (there is a single shot framing just the two of them).