William Hartnell made a career out of playing tough military NCOs both in films and TV. However in real life he joined the Royal Armoured Corps in WW2 but was invalided out within 18 months due to a nervous breakdown.
Forty or so minutes into the film, Terry-Thomas' character goes to watch In Which We Serve, where he finds half the company. Richard Attenborough plays one of the privates watching. Attenborough also happened to star in the film In Which We Serve