In the first episode, the character of Adam Klaus was played by Anthony Head. Although intended to be in every episode, the character was never seen in the rest of the first series. For the second series and beyond, Stuart Milligan played the role due to Head's involvement in Buffy the Vampire Slayer (1997).
The eponymous Jonathan Creek duffel coat actually belonged to Alan Davies. He wore it to set on his first day of filming and everyone agreed it suited the character perfectly.
Adam Klaus was originally to have been called Adam Faust, but this had to be changed when checks by the BBC's Legal Department revealed that there was a real-life magician with this name.
The windmill that Jonathan Creek lives in is called King's Mill or Vincent's Mill and was built in 1879 in the town of Shipley, West Sussex, around 6 or 7 miles South of Horsham, England. It was once owned by the writer Hilaire Belloc - a Frenchman with English citizenship. At various times throughout its history it was open to the public, but has been closed since July 2009.
Alan Davies has said that he was the 38th person to be interviewed for the lead role. Among those considered were Angus Deayton, Nicholas Lyndhurst, Hugh Laurie, Rik Mayall and Nigel Planer. Lyndhurst and Laurie turned the part down.
Although Anthony Head appeared as Adam Klaus only in Episode 1, he is seen as well in Episode 2 - as an Adam Klaus poster, hung on Creek's windmill's room wall.
The second series of "Jonathan Creek" was commissioned even before the first one was shown.
Several allusions are made to characters and locations from DC Comics. A famous magician who was once known as the "Black Canary" (a popular DC hero). A mansion called "Metropolis" (the city where Superman lives). And another estate called "Green Lanterns" (several DC heroes have used this name).
Jonathan Creek is a real place in Kentucky, USA which writer David Renwick discovered whilst driving through there with his wife and decided to store the name away for future reference.
The car Maddie Magellan drives in the series is a Volvo 240 GL Estate which had an automatic gearbox. The gear selector can be easily seen in the episode where Jonathan drives the Volvo for the first and only time.