Although they shared the same first and last names, the director Robert Stevenson was not related to Robert Louis Stevenson on whose 1886 novel "Kidnapped" the film was based.
Peter O'Toole was cast in the part of Robin MacGregor, because he had played the pipes in his youth and the director was looking for a tall young actor who could play the pipes.
The film takes place in 1751.
In addition to playing the landlord of an inn (who provides useful plot information), Jack Stewart has a second role in the film - he can be briefly glimpsed in a large wig and with a heavy beard as one of the clansmen protecting the outlawed Cluny, played by Finlay Currie. He has some dialogue in this role, but it's all in Gaelic (and it sounds like someone else's voice).