Without going to photography/film school... directing is a real challenge to me because there isn't a lot of options to learn the art. With technical skills there are a lot of tutorials on the web, directing needs a 'hands-on' approach. Most of it is learned by observing other photographers, or working as a studio assistant. Other times it's watching Behind-The-Scenes videos.
I think getting someone to be natural in front of the camera is very challenging for both the shooter and subject, and for the selected few images that need to be a specific look, working with the model to get that particular mood at the right moment is critical for success. It's also striking the balance between challenging your model to do something they're not used to, and backing off when you know they've reached their limits. Learning a good set of communication skills is just so important and portrait photography has changed the way I work with people.
BTS videos are useful - whether it is fashion/lifestyle/glamour/nudes. It documents the shoot/set itself, and demonstrates the photographer/camera man in relation to their subject/model. It gives you context to the scene, positions of all actors/crew/lighting as well as the overall mood of the set. They might also interview the model, the crew or the photographer/director to get their thoughts of the shoot.
NSFW Video of Lera Koryts'ka for World Swimsuit - This model is so beautiful and natural in front of the camera. Her interactions with her cameraman is fluid, easy-going and fun in that moment.