Although the author may have cast the character in a particular way, imposing that one specific actor/description on the reader is unnecessarily restrictive. Yes, the author may have worked hard to picture the scene s/he is trying to depict down to the specifics of hair and eye color, but contrary to the beginner's understanding of the process, the writer's job is not to reproduce that scene in the reader's brain
exactly as the author originally pictured it. On the contrary, one wants a certain level of vagueness, of blank canvas, onto which the reader may project their own experiences and preferences. Just as a playwright has to allow for a certain amount of interpretation of the script by the director and actors, the writer has to leave room for the reader to bring something to the project.
For example, if the story features a bully, then it is far better if in the reader's mind that bully merges with that bastard down in accounting who is currently making their life miserable. Of course their conscious mind is not about to suffer any such confusion, since we're pretty sure the guy in accounting is not in fact the murder or king of the space vampires, or whatever; but great fiction, like great opera, often bypasses the intellect and goes directly to the viscera, with people's emotions. The resonance between the writing and the reader's own experience may be disrupted, however, if one insists on establishing definitively that the guy in accounting is not the bully under discussion because the one in the book has red hair and blue eyes.
Or, to take an example from the other end of the emotional scale, if one is too precise in describing the love interest, one runs the risk of including a detail that is, for the reader, a deal breaker. "Electric blue eyes" is as likely to remind them of their ex as of their current lover. (It is the same reason why it seldom pays to be too explicit in sex scenes: if it doesn't happen to be the reader's kink, one is more likely to get an "eewww!" than a sale.)
So why go there? If the writer insists on determining every microscopic detail of the experience for the reader because that happened to be how the writer pictured the scene, then it's not about trying to be precise, it's about being a control freak. If one wants to build readership, one has to give up some control so the reader can take some ownership of the reading experience. If one wants readers to recommend the book to their friends, then the reader has to come to think of it as one of their books