Not often do movies trump books, but in this case, it might be true. To be fair, I love this book and my kids laughed out loud often, but the movie is better. Not because the lair under the tree was more enjoyable or because shrieking eels makes more sense then shrieking sharks, but really, because of Buttercup. Her character doesn't make sense in the book version. 

If Wesley is all that Goldman creates him to be, than why would he be so in love with such a dits? On this, the movie got it right. Make her pretty, but also make her smart, confident, and strong. Make her worthy of Wesley, and Wesley worthy of her.

The extent of their differences are spelled out here:

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Stealing from the Princess Bride