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.
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 - Im Bann der Dämonen (1997).
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 Paul Bown, Angus Deayton, Douglas Hodge, Peter Howitt, Alex Jennings, Nicholas Lyndhurst, Hugh Laurie, Rik Mayall and Nigel Planer. Lyndhurst and Laurie turned the part down.