I had an email saying, "I've done several screenwriting workshops and gained a lot from them. Are there any other courses you think are beneficial for writers?"
Yes! The more you have an understanding of the other aspects of filmmaking, the better. Here are the four I recommend:
ACTING: For finding out what actors look for in a role and what challenges they have saying somebody else's words, nothing beats taking an acting class.
Try to find a beginners' class, ideally one that includes others who want to learn about the craft without necessarily becoming actors. That will make you less self-conscious.
IMPROVISATION: You'll get something different from this than you'll get from a straightforward acting class.
Improv teaches you to think on your feet and quickly immerse yourself in a character. It also stops you from being too precious about any one idea, because you learn that ideas are endless. And it's fun!
EDITING: Find a course that teaches the basics of editing--not the software, but the effects of juxtaposing two things, of short scenes vs. long ones, and of the importance of reaction shots. Filmmakers know the extent to which editing determines whether a movie works or not.
Once you appreciate the basics, you'll find that you're reflecting them in the way you structure your scenes on the page.
DIRECTING: Just as it's helpful to experience what actors look for, the same goes for stepping into the shoes of a director, even if you never intend to direct. You might even discover you have an affinity for it and become a writer/director.
That's a lot but there's no need to rush into learning it all.
If you can do only one, I'd go for an improv course. To get a taste of how relevant the principles of improvisation are for any of the theatrical arts, read Keith Johnstone's book, Impro: Improvisation for the Theater (one of my favorite books for its insight into human nature).