You say “thank you.” That’s it.
You remind yourself that this person took the time to (a) read your work, (b) give it enough thought to formulate comments, and (c) took the time to share those comments with you. You don’t get defensive or tell the person that he missed the point, that he doesn’t understand good writing, that he obviously wasn’t paying attention, that he doesn’t know or understand your genre, or that you never realized what an idiot he is in general until now.
If your reader asked any questions, you can answer them. If you are unclear about a criticism, you can ask for clarification. If you want to bounce a potential revision off him and see how he likes it, you can do that. And if you must – absolutely must – you can explain what you were trying to achieve with the work. Under no circumstances, however, can you tell the reader that he or she is wrong.
Every reader’s experience and interpretation is unique and personal. There’s no such thing as a “wrong” read. Writing is, after all, a direct one-to-one form of communication, from the writer to the reader through the printed page (or screen). If the reader isn’t getting what the writer intends, then it is up to the writer to hone his or her craft, not berate the reader for missing the point.
I bring this up because it became a bit of an argument between several members of my writers group and a new member who felt that writers are entitled to argue criticism. I feel pretty strongly that arguing with a critic is discourteous, inappropriate, and immature.
The writer, after all, holds all the cards – he or she decides how to revise. If you, as a writer, don’t like a suggestion you receive in a critique, you have the power and the right to disregard it, act as if you never heard it. What you don’t have the right to do, if you are mature about your craft, is tell the reader what a terrible suggestion they have just made. You keep that to yourself. You remember that criticizing another person’s creative work often takes almost as much courage as offering your own work for criticism. You remember that this person gave you the gift of their time and attention, and their comments are meant for your benefit.
And you say “thank you.” And if you really are angry about it, you base an especially unpleasant character on that critic and kill off the character in a really awful way.