Deborah Kerr & The Screenplay Are Just Slightly Mismatched...

The screeplay makes several references to Deborah Kerr's character , Miss Giddens, being "a child" and taking on her "first position" as a governess and so forth, all of which gives the impression that the governess is intended to be far younger than Kerr, who was around 39 or 40 when the film was shot. Miles makes several comments about how 'pretty' she is, rather than using the more mature term 'beautiful', which reinforces the point. However the obviously repressed nature of the governess, an over protected vicar's-daughter, corsetted by rigerous Victorian conventions of purity and it's inevitable furstrations, works far better if the character had been of Kerr's actual age. The impending 'old maid' in fact.
It's a small point in itself but if the numerous references to the youth of the charachter had been edited out of the script it would have worked just that bit better and the unsettling pent-up desire that she obviously posesses would have been even more keenly underlined in that aspect of the story - which in the view of many is the core of the whole piece.
The alternative would have been leaving in the references and casting a different, much younger actress. But then who would have been suitable for the role in what was a major film of it's day, when Kerr was a bankable worldwide star?