When Peter Jackson originally cast Ryan Gosling to play the father role in his adaptation of The Lovely Bones. Just a few weeks before the movie was scheduled to start shooting, Gosling was let go and recast by Mark Whalberg.
Reports at the time varied as to what the reason was for the recast. The most circulated explanation was that Jackson thought Gosling was too young to play the part. Some other people said that Gosling had been too demanding and that made working with him difficult.
Later, however, Gosling himself explained the real reason for his firing. Gosling gained 60 lbs for the role because he thought that would be the best direction for the character. Jackson disagreed and he was fired.
In case you were wondering, Gosling's secret for getting fat? Drink melted Haagen Das instead of water.