He assisted people such as Mischa Spoliansky on The Ghost Goes West (1935), Arthur Benjamin on Wings of the Morning (1937), Anthony Collins on Sixty Glorious Years (1938), Nicholas Brodzsky on Freedom Radio (aka A Voice in the Night, 1941) and Tomorrow We Live (aka At Dawn We Die, 1943), Noël Coward in In Which We Serve (1942), John Ireland in The Overlanders (1946), and Walter Goehr in Great Expectations (1946).

They went on to work on such films as Victoria the Great (1937), The Lion Has Wings (1939), Gaslight (1940), Old Bill and Son (1941), Dangerous Moonlight (1941, which contained the famous Warsaw Concerto), Love on the Dole (1941), This England (1941), This Is Colour (1942), The Big Blockade (1942), The Day Will Dawn (aka The Avengers, 1942) and The New Lot (1943).

Some sources suggest Addinsell had good musical ideas but no skills in orchestration, and that Douglas's role was much more than a mere assistant or copyist.

[2] Douglas and Ernest Irving helped Walton complete the ballet The Quest by his birthday, 29 March 1943, in time for its premiere performance only a week later.

[5] The vocal score of Troilus and Cressida was largely the work of Roy Douglas, assisted by Franz Reizenstein in Act III.

[10] For example, he was first made aware that RVW had written his 6th Symphony in a letter from the composer dated 13 February 1947, but he was not given the score to work on until almost seven months later.

[8] In an apparent departure from the usual method, Douglas was asked to write out the score for the Tuba Concerto in 12 days to meet a deadline, but without the opportunity of checking with RVW's piano sketches.