I happened across a question someone asked on Reddit about what to do when you get Stuck in the course of Writing a screenplay. The following answer is by "Hotspur Jr", a member of the Writers Guild of America (I'd credit him or her with their real name, but I don't know it):

This is what they do when they get stuck on how to write a particular scene:

"Write down what you know.

I need the scene to do X, Y, and Z. I need the scene to do blah. I'd like these three characters to be in the scene. The scene needs to feel like this. I'd love it if she said a line like "this". It can't happen at the beach house because of X. It needs to feel different from the car scene. The scene is difficult because of Z.

Just keeping writing down everything that you know about the scene. I do this in an open OmniOutliner document.

And at certain points, I'll just start sketching the scene out in an outline document. What would THIS version of the scene look like? I rough it out, and then look at what I roughed out.

Why does this not work? It doesn't work because of X and Y. What if I changed X to Z?

Rough it out again.

Repeat as necessary.

There are two things this does.

The first is that it takes that vague feeling of "This isn't working," and it breaks into concrete problems that I can fix. It takes what's in my subconscious and lets my conscious mind take a crack at it.

And the other thing it does is occupy my conscious mind with concrete tasks so that my subconscious is free to zoom around, put weird ideas together, and come up with aha! out of the box solutions.

It's also a scientifically proven method for trying to solve difficult conceptual problems: don't try to solve the problem. Just talk about the problem without specifically aiming for a solution. In this case, the "talking" is the conversation I'm having with myself by writing stuff down. (Writing it does is important. What you write down is not, but the fact that you're writing it down stops you from going in loops).

Lastly, there's one more step, and it's very important. After I've done that for an hour or two, I go do something else. Play a game of chess. Go for a walk. Pick up my guitar. I mean, why do y'all think I post on reddit so damn much?

Something that takes my conscious mind off the problem entirely. This is, again, a tool that is scientifically proven to work."

Good advice!