[4] Marsh mentioned the embryonic novel in the 1981 edition of her autobiography, Black Beech and Honeydew: "I have often dallied with the notion of writing a book about a company rehearsing Macbeth, which, as every actor knows, is thought to be an unlucky play.

In 1981, she wrote to her close friend John Balfour that the novel had been in her mind for a long time, was "hell" to write and would, she thought, appeal to theatre people rather than to her usual fans.

[7] John Coleman in a capsule review for The Sunday Times called Light Thickens the "Last, honest bow from the mistress of the clued-in genre", though he made gentle fun of the fact that Alleyn was still a serving police officer after a professional life of over 60 years.

"[1] Anthony Lejeune in The Daily Telegraph called the book "agreeably self-indulgent", noting that more than half of it passes in a detailed account of a rehearsal of Macbeth before a murder occurs.

[12] Light Thickens features a hard-left Equity representative who attempts to politicise fellow cast-members and who secretly belongs to an organisation called "the Red Fellowship": Bruce Harding describes this detail as "anachronistic", comparing Marsh's depiction of a Communist character in The Nursing Home Murder (1935).