Dear Mr. X, I hope you can understand if I decline your offer. You see, I feel it is sort of like asking Meryl Streep to audition for a Sharknado sequel. I'm not saying she wouldn't agree to do the movie - I wouldn't dare to speak for Meryl - but asking an actor of her experience and caliber to audition for a film her repertoire shows she is clearly capable of handling is just poor form.
For most people who write for a living, it’s not all sonnets and short stories; you’ve got to sell a bit of yourself. Or a lot of yourself, depending on the project. Is it worth it?