What's that about?
I think a lot of what a sketch has that more carefully produced paintings lack is an overall cohesiveness that comes from consistent brushwork. In the sketch below, there is a convincing feeling of strong sunlight that comes from an accurate reading of the values, but there is something else at work that contributes significantly to the sense that everything on the page comes from the same world. The brushwork all feels swift and uncorrected, as if it didn't matter whether that piece of paper turned out to be a success.
Can you do that on purpose?
On a good day, you can, but it's like the old zen conundrum: If I strive not to strive I am striving.
In the painting below, I shifted gears between layers and ended up with parts of the brushwork feeling very different from each other. Those hard-edged wavelets in the left foreground feel quite separate from the general statements about the water, as if theywere scraps of fabric floating on the surface. Similarly, the trees on the hill in the background have a different feeling from the hill. They are just a bit frantic, while the hill itself is very calm. The color choices almost pull it all together, and a glaze over the trees might help there, but the foreground would still need some first aid.
What do you think about this one? Is it cohesive?
For homework, choose one or more of the photos that follow and make very quick studies, no more than ten or twelve minutes, and don't correct them! Really. If something happens that you hadn't intended, let it be and keep moving on. There are some very useful lessons to be learned about the range of what's acceptable.
Keep it approximate. Ten minutes doesn't give you time to be fussy.