I imagine all the scenes in my head as I'm writing. Since my source material is a movie, it's pretty easy for me to do.
I think I do the same thing (all the fandoms I belong to are TV shows, so yeah, imagining scenes is pretty much how it goes). I think I just write down the dialogue first because otherwise I have to mentally rewind and catch it all again... or maybe my brain is just broken...

I don't know how much interest there will be since it's small fandom and I have an OC, but I'm not posting any of it until it's finished.
A little bit off-topic from the rest of the discussion, but I will tell you that I am weird about reading stories with OCs. I have no problem with OCs in general - sometimes you need to make up characters to keep the plot going - but the larger the role they have, the more resistant I am to them. Like, yeah, if it's a minor character that Sylar needs to kill, that's fine. If it's the latest scumbag that Veronica Mars is investigating, that's okay too. But when they become a main character's romantic interest or something, I'm not interested.

I also have been avoiding OCs in Glee fics like they are the plague, because every time I've given one a chance they've turned out to be a total Mary Sue.

I don't know if that's at all reassuring or not...