"Listening.
When I started improvising, it felt like the task was 50% listening, 50% doing stuff, deciding. Your partner does something, then I do something. Seems sensible.
But the longer I've improvised, the more I've realized that the whole thing is listening. The more you listen, the more you notice. The more you notice, the more you can't help but react. And if you can't help but react, your choices are already being made.
In a good scene, I don't feel like I'm deciding to do anything. Everything is being discovered, from my partner, what they said, how they said it.
Or from listening to myself. How am I standing? Why did I use that tone? How do I feel?
The scene is the thing there happening, not the things that could happen. If I just pay attention to what is happening, what it's making me think, that it's making me feel, then the scene is already right there.
Good scenes feel like you're just plugging your brains in together and going along for the ride; giving over to the play of the moment."