There is a copy of Northanger Abbey available for reading online at
http://www.gutenberg.org/files/121/121-h/121-h.htm#2HCH0001In showing actual "scenes" from Catherine Morland's imagination, I think that the TV version did emphasize her flights of fancy more than the book does, but I think that the production is true to the spirit of Jane Austen's work. Catherine definitely has set herself up to see melodrama at Northanger Abbey.
From the book:
Catherine has just confessed to Henry Tilney that she has been in his mother's room. He says:
" 'If I understand you rightly, you had formed a surmise of such horror as I have hardly words to—Dear Miss Morland, consider the dreadful nature of the suspicions you have entertained. What have you been judging from? Remember the country and the age in which we live. Remember that we are English, that we are Christians. Consult your own understanding, your own sense of the probable, your own observation of what is passing around you. Does our education prepare us for such atrocities? Do our laws connive at them? Could they be perpetrated without being known, in a country like this, where social and literary intercourse is on such a footing, where every man is surrounded by a neighbourhood of voluntary spies, and where roads and newspapers lay everything open? Dearest Miss Morland, what ideas have you been admitting?'
They had reached the end of the gallery, and with tears of shame she ran off to her own room.
...
The visions of romance were over. Catherine was completely awakened. Henry's address, short as it had been, had more thoroughly opened her eyes to the extravagance of her late fancies than all their several disappointments had done.
...
Her thoughts being still chiefly fixed on what she had with such causeless terror felt and done, nothing could shortly be clearer than that it had been all a voluntary, self-created delusion, each trifling circumstance receiving importance from an imagination resolved on alarm, and everything forced to bend to one purpose by a mind which, before she entered the abbey, had been craving to be frightened. She remembered with what feelings she had prepared for a knowledge of Northanger. She saw that the infatuation had been created, the mischief settled, long before her quitting Bath, and it seemed as if the whole might be traced to the influence of that sort of reading which she had there indulged."