It is rather devoid of women, so I'm glad that Charles Bennett added a few, including the glamorous Pamela for the dashing Donat and all that romantic handcuffing subplot stuff in the classic Hitchcock screenplay of 35. In fact, Bennett's other additions are quite remarkable for being film savvy but complementary to the spirit of Buchan and his protagonist. Loosely based in this case means subtle developments of loose images or lines in the novel, or new fabrications that match.
I posted this when I last saw the film, and re-visiting it found this essay The Hitchcock Universe: Thiry-nine steps and then some which discusses the adaptation:
17 Further notes on film adaptation. Although it is obvious to point out just how greatly Hitchcock's movie differs from (and improves upon) John Buchan's novel, I find it intriguing to note how the screenplay writers ... may have been inspired by phrases, sentences and images in the novel:
another comparison here
Though I knew Hitchcock seldom brought any book to the screen without "Hitching" it up quite a bit, I wasn't prepared for the extent that Hitchcock and his frequent screenwriter Charles Bennett had gutted the Buchan novel and fabricated all-new contents.
The book and its adaptations (including the latest by Robert Towne) are discussed in this Guardian article
In the public's mind, Hitchcock's movie and Buchan's book have become as intimately entwined as Donat and Carroll in handcuffs on the moors. No subsequent film-maker has been able to escape Hitchcock's shadow
Interesting to read that this lastest version has been gestating for a very long time. Back in the late 1990s, Warner Bros tried to make it.
(For a time it seemed that Mel Gibson was to be Hannay in a downunder-outback production - would that be spaghetti haggis?)
An extract from Charles Bennett's memoirs
At a cocktail party celebrating my 47th birthday, Hitch wrote a facetious remark into my party book, 'You owe everything to me, including your love life!' Everything? Hardly! By the time of The Man Who Knew Too Much (1934), I had staged seven plays and directed five, written more than fifteen films, and had succeeded as an actor! Arguably, Hitch owed me! My Shakespearean experience and sense of melodrama built his reputation for suspense. My play BLACKMAIL was his 'FIRST SUPER TALKEE'. My story "The Man Who" launched his international reputation. My construction of The 39 Steps put him out front by inventing motifs he would use in subsequent films. My recommendation to David Selznick helped bring Hitch to Hollywood. Etc., etc. ...
But Hitch would not acknowledge it. It always had to be Hitch! He would not acknowledge any writer. A very ungenerous character flaw, actually, as Hitch was totally incapable of creating or developing a story. Without me there wouldn’t have been any story. Hitchcock was never a constructionist, never a storyteller. I would take a story and turn it into something good.
Realism coloured by poetry: Rereading John Buchan