The first reason: the actor is not suitable for the role.
Some novels are adapted into movies because directors and screenwriters value the influence of novels, but they don't really want to make them into movies. There is always an actor corresponding to the role in the hearts of original fans. If the production company doesn't do a good job of investigation in advance, it only asks the traffic star to be the protagonist, regardless of whether the star's image and acting skills have completed the role in the novel. Then when this happens, the original fans will strongly resist the adaptation of this novel into a movie.
The second reason: the phenomenon of magic change is rampant.
Fans like to read this novel because the scenes and characters described in the novel are very pleasing, otherwise the novel will not be liked by fans. However, there are many movies or TV series adapted from novels, and there will be magic change. This probability is about 9/ 10, and we can also think that magic phenomenon is widespread in the entertainment circle. There are even many protagonists whose plots are arranged in supporting roles, which is why the original fans strongly resist.
The third reason: the supporting role plays at will.
Original fans like to watch their favorite novels adapted into movies according to the plot of the novels, so original fans hate supporting roles and scenes very much. The Untamed had a supporting role and a play, which was unanimously boycotted by the fans of the magic master. In the end, none of these scenes were deleted, and even the supporting roles in the play disappointed the fans.
If every adaptation will retain the actors sought by the novel plot, which is very in line with the character's personality, and the supporting actors will not add plays at will, then the original fans will not resist the film.