When she is a double agent in The Lady Vanishes of course!
The nun in question, was played by Catherine Lacey. The actress, born 6th May in 1904, appeared on stage several times during the early part of her acting career and made her film debut as the high heel wearing nun in Hitchcock"s The Lady Vanishes in 1938 (pictured).
In 1949, she played the part of Mrs Waggett opposite Basil Radford (of The Lady Vanishes fame) as Captain Waggett; she reprised her role in 1958 in a little known spin-off titled Rockets Galore – on this occasion Captain Wagget was played by Roland Culver.
Other films that she appeared in include; Cottage to Let 1941, I Know Where I'm Going 1945, Pink String and Sealing Wax 1945, The October Man 1947, The Man in the Sky 1957 and The Servant 1963. In 1968 she was awarded a Silver Asteroid award for Best Actress by The Trieste Science Fiction Film Festival for her role in Boris Karloff's The Sorcerers.
Her final film appearance was in 1970 in The Private Life of Sherlock Holmes although she appeared in a small number of TV productions in the early 1970’s. She died in 1979.