The way actors practice their lines before a performance, that's why it is good to practice painting a subject before you work on the final painting.
In a finished painting, there are many questions that you must to ask yourself, and often, many answers to those questions.
Let's say you're painting a leaf.
You may ask yourself, "What is the shape of the leaf? Is it long and narrow, or short and wide? Does it have multiple points, or just one? Are the edges smooth, jagged or lobed? Are the veins visible? What colour is it? Are there many colours, or only a few? What does it look like from a different angle?"
It is good to set down the answers in the form of painted sketches, until you are fluent in the language of that one leaf.
Having a dress rehearsal is important if you are unfamiliar with the script.
Actors who improvise do so from a place of utter familiarity with their material.
It is only when we are fluent that we can begin to confidently break the rules.