Dwarves are an important part of the mythology for Middle Earth that J.R.R. Tolkein created with his Lord of the Rings series.
Even though Jackson’s Lord of the Rings series is widely considered a near-perfect trilogy, Hollywood had yet to discover a good concept that it doesn’t try to run into the ground.
Actors generally try to give their best performance with every role, but how the movie turns out is never really in their hands.
A similar thing happened to Cate Blanchett on the sets of Lord of the Rings, where, in addition to playing Galadriel, she also pitched a second secret role to director Peter Jackson in order to hang around the sets longer.