If the movie director is not giving you lemons, there is nothing you can do. The lemonade principle says that you have to wait for lemons so that you can make lemonade.

If the movie director is not part of your personal network, then you should not trust him. He is probably just trying to get rid of you so he can steal your idea.

The lemonade principle says that you have to squeeze out every opportunity until you are absolutely sure that there is no value in it. When you have lemons, to get lemonade you have to squeeze them really hard. So the lemonade principle suggests that you should not listen too much to this one film director, and that you should continue to trust your gut and develop the idea as you had initially planned.

If the movie director is giving you not only the existence of a need, but also a willingness to pay, you may have identified an underserved market. The lemonade principle suggests that this is a surprise that you should capitalize on and see if there is more interest in the idea, whether you have the skills and knowledge to develop it, and whether you have an interest in taking it further too.