Before filming the scene where Robert Donat and Madeleine Carroll run through the countryside, Alfred Hitchcock handcuffed them together and pretended for several hours to have lost the key in order to put them in the right frame of mind for such a situation.
The 62 imported sheep, upon arriving at the sound stage, immediately went to work on the bracken and bushes that had been brought with them. The infuriated crew had to replace the real plants with ones hastily bought from a local nursery.
The failure of the original copyright holder to renew the film's copyright resulted in it falling into public domain, meaning that virtually anyone could duplicate and sell a VHS/DVD copy of the film. Therefore, many of the versions of this film available on the market are either severely (and usually badly) edited and/or of extremely poor quality, having been duped from second- or third-generation (or more) copies of the film.
The scene at the farm at 30:00 from the start, bears striking resemblance to the first scene of the opera The Walküre from Richard Wagner. Hitchcock must have seen the opera. The unhappy farmer's wife, the suspicious husband, the newcomer. The glimpsing over the table back and forth, the sexual tension.
The bridge on which the train stops to search for Hannay (25 minutes from the start of the film), is the famous railway bridge over the river Firth of Forth, build from 1884 till 1889. It was then one of the most complicated works of engineering craftsmanship. It spans 2,5 km.