Thor will fetch if the price is right. I have to give him a treat, even though I've explained to him multiple times that dogs are supposed to fetch for free.
I usually have him do a series of tricks, or at least have him drop it directly in my hand for a reward. If he just drops in on the ground, no treat.
I've tried to put it on a random recinforcement schedule, but Thor is not having any of it. If he fetches three times without a treat, he will deliberately put the ball somewhere out of reach, and walk away to teach me a lesson.