We are the mayors of now-town.
This is a lovely phrase summarizing a lot. The moral is that we have to stay in the here and now. Not only with our attention and focus, but also with every line of our dialogue: say sentences that live in the now.
"You have no idea what I went through" lives in the past, "There's so much work to do" lives in the future, but "I'm enjoying this so much" lives in the now.
If you're having a hard time staying in the now, you're probably plotting too much. Plot has never saved a scene!
Don't move forward too much. Improvising a scene is like inflating a balloon together. Of course we want it to pop eventually, that's what changes the characters. But don't force pop the balloon. Then your partner will not know what to do.
You don't want to taste the cake while it's still in the oven, right?