Rose Byrne did all of her own piano playing.
Manorbier Castle in Tenby, Wales, which was used for many of the shots of the Mortmain family's castle, had a moat that had not been used for many years. For the shots of Cassandra and Neil swimming in the moat, large amounts of clay were brought in to make the moat watertight. It was then filled from tankers of water brought from elsewhere, because the local water supply could not supply the large amount needed.
The Castle used on location in this film was Manorbier Castle, Pembrokeshire, Wales, Great Britain; (but not the shots of the lone turret). Manorbier Castle was also used as location for the BBC adaptation of CS Lewis' "Prince Caspian"
This film is based on the first novel by Dodie Smith. It was published in 1948, when she was 52. She had previously written several popular plays and had worked on film scripts, some in Hollywood.
This movie is supposedly set in Suffolk; Henry Cavill later starred as the Duke of Suffolk in The Tudors.