Liz Fraser replaced Janet Davies because Norman Cohen wanted a less homely, more "sexy" actress for the role. Jimmy Perry has said "It was a mistake...not to cast Janet in the role because the viewing public has come to recognise her as Mrs. Pike. But that was a decision made by Columbia."
Arthur Lowe had a clause written into his contract which stated that he would never appear without his trousers on. This is why Sgt. Wilson is leading the platoon during the platoon's changeover march (when the platoon change from civvies to uniform via their long-johns) instead of Captain Mainwaring. (Incidentally, this is also the reason why Jones gets the grenade down the trousers in the U Boat crew episode - it was originally going to be Captain Mainwaring).
Neither the cast nor the writers were happy with the film.
Whilst the television series filmed its exterior scenes on location around the village of Thetford in the county of Norfolk, this film did not utilize the same area and instead filmed its location scenes in the Buckinghamshire countryside, about 80 miles away from Thetford. This was presumably done because it was much closer to the production offices at Shepperton studios in Surrey.
The tweed jacket worn by Arthur Lowe was subsequently used by Kevin McNally to play Captain Mainwaring in an episode of Dad's Army: The Lost Episodes (2019) - this was not planned, as costume designer Howard Burden needed an appropriate jacket and simply got the first one he could find from Angels Costumiers that looked right and would fit the actor.