How much control does the author have over the movie?

Question

I was watching the movie "The Princess Diaries" recently and it got me wondering how much control does the author have over the movie adaptation ?

The movie and the books digress on the most pivotal plot-lines like (i read all the books and the side books of this series)

1. Boyfriend of the princess is one of the main plot of the books but hes not even a part of the 2nd movie (in the book, the princess actually makes a sarcastic reference to the movie breakup)
2. Father of the princess is dead in the movie, but alive in the books
3. Movie grandma is good vs book grandma is evil.. etc.
4. the part 2 is not even based on the books

So when major changes like this happen in the movie, does the author have a say in it?

Answer 1

So when major changes like this happen in the movie, does the author have a say in it?

Usually, none

It depends on what was in the contract to acquire the screen rights. Some retain control, some don't.

JK Rowling did...as explained in the M&TV Q&A

Generally the book author just sells the film rights and walks away. They may be asked or even be contracted to convert the book into a screenplay but this is often not the case.

Another example of an author exercising some control is Pamela "P. L." Travers who wrote Mary Poppins but insisted in script control from Disney but even that was only partially successful.

Travers so disliked the Disney adaptation and the way she felt she had been treated during the production that when producer Cameron Mackintosh approached her years later about making the British stage musical she acquiesced only on conditions that English-born writers alone and no one from the original film production was to be directly involved.

^Wikipedia