When I first began writing, it was with the intent to write screenplays. I had no idea whether whatever story I was concocting would even be best told in a film. I chose the medium first and tried to fit whatever story I came up with into that medium. Screenwriting is limited in that all that will make it into the finished product, a movie, will be sight and sound. There are certain story turning points that can work perfectly well in a novel or short story that don't make best use of the cinema's inherent qualities.

He Realized

In a novel this can be a profound moment where the character has seen something happen and draws a conclusion based on it. It could shock and confuse the character and push him or her in a different direction pushing the story forward. In movies, characters don't realize. Realizing isn't something you do physically so actors never realize. There are contrived faces an actor can make to look like they just had a realization but we will never know for certain and won't know the precise nature of this realization if we do assume one has transpired.

He Knew

In a novel you can come to know the psychology of a character intimately. You can learn directly what they know and what they don't. They can simply tell you something that they knew or just learned and it can point the story in a different direction. It can also be used to motivate the action of a character that wouldn't make sense otherwise. In movies we don't know what people know, we only see what they do and hear what they say. Any assumption of what a character knows is just that: an assumption. This is something you came up with, not something the author necessarily even meant for you to infer.

He Thought

Along with being privy to what a character knows, in novels you have immediate access to character's thinking. Their process of how they figure things out can be the bulk of a novel and you can have front row seats to what they think and how they came to think it. In movies we don't see thinking. We can see furrowed brows, pursed lips and looks to the heavens; but we never can see thinking. Thinking is not an external action and we can only approximate it by having an actor perform facial movements to elicit the idea that the character is thinking.

He Decided

In a novel a decision can be the climax of a scene. It can be the ending of a book, it can be the thing that a reader was looking forward to the entire time, and it can be the culmination of all the events thus far. In movies people don't decide things. Deciding is an internal discourse and can't be viewed. You can see a character do something but you don't ever see them decide to do something. Thought an actor could make a scale with palms up and pretend to weigh two different ideas, this is an inelegant device that is better left to prose.

Instead of choosing your medium and forcing whatever story you come up with into it, come up with your story and put it in the medium that it best fits in. Give yourself time to expand upon the things you think are most visual and deserve to be in a movie rather than trying to visualize essentially internal processes so they can play on-screen. For maximum impact a medium is best used for it's strengths and not it's position in popular culture.