Preparing to be spontaneous......
As weird, as it sounds spontaneity can be planned.
I do this is with acrylic painting. I often paint with acrylics on watercolor paper.
Most of my watercolor paper is 140 lb weight cold press. By the time I am done working it is usually at least twice as thick. This is because it is coated with gesso prior to painting.
Gesso comes in many forms, thin, thick, grey, white, black, colors and clear. If you only have white, it can be tinted with a color, or you can get colored gessoes from Holbein (China red is my favorite), or Daniel Smith (gold is really fun to use).
A basic layout or composition plan can also be started using a couple of colors of gesso.
Using gesso with color knocks out my two issues right at the start. I spread the gesso thickly with a spreader leaving scrapes and ridges randomly and generally use a mid tone color so that all I have to focus on are my lights and darks when painting.
I have started making my own spreader. Plastic snap on lids from food containers work great, and you are recycling at the same time. Clean the top off, then cut it in half, I also trim the corner edges a bit and they are ready to go.
These prepped sheets are set aside until I get in the mood to start painting, or as it really appears, dive into a spontaneous mess of splashing paint around.