This is short and sweet: for me, pre-production is the most important part of the whole process. This is where I discover and plan for any problems I might face when making the film. At this point, I've already rewatched the film over and over in my head 100 times. I know what I like and don't like about it. I see what limitations I might be facing. And I redirect in order to best get my point across once I start filming. I get over the "I love this scene, I can't wait to shoot it" and into the "but this scene doesn't work as well, how do I fix this?"
It's easy to get caught up in the infatuation you have with the overall essence of the idea and jump too quickly into trying to capture it. At this point of the process, patience and being completely savage on yourself is the only way to get it to a point where you can be sure you've done the work to make this thing come to life. It's hard and I'm sure others may view it from the outside as obsessive. Every scene should have purpose, and there are small details that help amplify this. I want to have that all figured out before filming. Once you get to the point where you can watch the film in your head and you don't feel the need to skip through scenes or feel weakness in certain parts, I'm confident I can start shooting.
This approach by no means is the "right" approach. My approach works for me with smaller budgets and limited shoot times. Fellini basically made up 8 1/2 as he went along and wrote much of the dialogue in post, and it's one of the greatest films ever made. Kubrick could spend weeks blocking a scene, even though he meticulously planned his films. On A Clockwork Orange, it took nearly a week to get to a fix for the "Singing in the Rain" scene. Each film and director will have their own approach. I like to create a sense of predictability as much as possible. That way, when the unpredictable pops up, you have more time to focus on it.
One thing I like to do is film "mood boards" on location during this phase. I start working with my lensing and doing camera tests. I watch what the environment is doing. I look at the natural lighting and see what I like and don't like. I go at different times of the day so I know how the sun will hit outdoors. I take some video and photographs and start composing some of my key shots. I then cut this into a short 3-10 minute video and share it with the composer so we can start working on the feel of the music (I like to listen to some of the score before filming). Some of these shots might even make it into the final film, but in general they're educating me on how to approach my shot list given the environments I'm working with. They're like my sketch book. For my short Main Street, they were key because it was important that the environment was a central character in the film.
Here's an example of a mood board I made for Kitsune:
And an example of a mood board from Main Street:
At this point you can see that I've started developing the overall aesthetic of the film. I don't normally ever share these, but I think it could be helpful for other filmmakers who might find this useful for their own use.