Alfred Hitckcock's 1935 thriller The 39 Steps is consistently listed as one of the best novel-to-film adaptations ever. That's interesting, because John Buchan's adventure novel The Thirty-Nine Steps isn't that great of a book -- lots of plotholes, incomprehensible coincidences, and the titular steps themselves are a literal staircase on a beach. Buchan, the 1st Earl of Tweedsmuir (seen here cosplaying as The Apache Tracker) would become Governor General of Canada the year Hitchcock's adaptation came out. Hitchcock would make his way across the Atlantic a few years later, after making The Lady Vanishes.
Anyway, Hitchcock fixed most of the problems with the book, changing it substantially, while also adding a few of his own. Lost in Criterion discusses them, and the rest of the film, in this week's episode.