What I've found is that the more time one spends in front of the subject, the more information is absorbed, albeit much of it unconsciously: direction of light, color of light, shapes, and so forth. Taking reference photos is also important, another source of details. Pencil thumbnail drawings and an 8x10 study force one to decide what should be included in the painting and what to leave out.
My painting partner and I hiked for almost an hour assessing subjects on a friend's farm, rejecting one scene after another.
This study was painted in full sun and is too dark and the colors too pale. But the composition made it worth using for a larger painting. All kinds of trees, bushes, etc were left out. Studying the reference photos allowed me again to pick and choose, and invent, additional detail and also deepen the shadows and intensify colors.