Here’s my “be there” take. I think that this is something every writer struggles with nearly every time he/she writes. You have to be there in the scene you’re writing. You have to write it from the inside out and not the outside in.
You may be the kind of writer who makes charts for your characters, outlines, writes diary entries, makes maps and timelines, does a ton of research. Maybe you just write intuitively. Doesn’t matter. Writers have different ways of getting into manuscripts. Robert Olen Butler does a ton of prewriting. My friend Cynthia Leitich Smith writes a first draft and throws it away and starts fresh. Most of us write very rough first drafts, and the only way we get through them at all is to keep telling ourselves they’re just a rough, rough sketch. It will get better.
Still, when you’re actually writing you have to find your way to the place that allows you to be your characters and your characters’ world. You have to see and feel and experience their world with them. If you can get there, you have a better chance of choosing the right words. Your characters will be more likely to do the things they’re meant to do within the context of the universe you’re creating, and your plot will develop in a way that grows out of the characters and their situation. You’ll make fewer wrong turns and bad choices. Conversely, if you find yourself forcing your characters to do things you’re often going down the wrong path. You’re outside the story instead of inside.
Maybe it’s a little like acting. Of course the actor has to know the movie, learn her lines, be directed in scenes, work with other actors. But when it comes to acting, to actually making the character come alive, they have to be that character, be in that character’s skin or they won’t be convincing.