Kenneth Williams appeared to be in two minds about Carry On. He complained that the cast were poorly paid, and considered the films to be beneath him; he was quick to find fault with both his own work and that of others. Even so, he spoke fondly of the series in interviews. Wikipedia quotes Peter Rogers, the producer of the series: "Kenneth was worth taking care of because, while he cost very little – £5,000 a film – he made a great deal of money for the franchise."
Wikipedia also notes that "In his diaries, Williams wrote that he earned more in a St Ivel advert than for any Carry On film, although he was still earning the average Briton's annual salary in a month for the latter."
The five Carry On films that Kenneth Williams didn't appear in were Carry On Cabby (1963), Carry On Up the Jungle (1970), Carry On Girls (1972), Carry On England (1976) and Carry On Columbus (1992).
Carry On Cabby wasn't originally conceived as a Carry On film, but mid–way through development a decision was made to include it in the series. Williams was offered a role, but turned it down, believing the script to be inferior. The role was taken by Norman Chappell, who appeared in minor roles in many other films and television programmes of the time, including some other Carry Ons.
According to Wikipedia, the role of Professor Tinkle in Carry On Up the Jungle was written for Williams, but he turned it down as it clashed with filming for his TV show International Cabaret. The role was taken by Frankie Howerd. (This was Howerd's second and last appearance in Carry On, after Carry On Doctor (1967) – when he received good reviews as a quack doctor who becomes a reluctant patient).
Also according to Wikipedia, Kenneth Williams was not available to appear in Carry On Girls. He did star in two West End stage productions around this time, and also had various commitments in television and radio.
Wikipedia also reports that the only Carry On regulars to appear in Carry On England were Kenneth Connor, Joan Sims and Peter Butterworth (the last two in minor roles).Williams died in 1988, four years before the start of filming on Carry On Columbus.
© Haydn Thompson 2021