If you want to improve your drawing and painting, take time to focus on your subject from an academic perspective. Although it is essential to play and experiment, it is also important to dedicate time for disciplined study to learn more about your subject. In this example, I focused on the mouth. However, this type of practice will help you better understand the form of any subject.
I worked from a photograph for this study. I added cross-contour lines to better understand the dimensional form of the mouth. Cross-contour lines are lines that follow the surface of the form. (In contrast, contour lines follow the edge or boundary of the form like an outline). Cross-contour lines often do not physically exist, although they can. For example, a striped object literally has lines on it that follow the surface of the form. Also, a shadow cast across an object can reveal the contours of the surface of that object. In order to convincingly create cross-contour lines, I had to consciously analyze the structure of the mouth and visualize the flat photograph as a 3-dimensional object.
Typically, I prefer to block in simple value shapes and build up my drawing or painting by refining shapes and values from general to specific. (I have several posts that demonstrate that process including this Mouth Study). However, sometimes we don't get the information that we need from a photo, or we have a limited time with a subject from life. Whenever I get stuck and something doesn't look right, I analyze the structure of the object to see where my shapes do not align with the structure of the form.
Light and shadow shapes are often more forgiving. Cross-contour lines are not so kind. When the lines do not convincingly follow the surface of the form, they just do not look right. That is what makes cross-contour drawing such a great exercise. You can't fake it, so you really have to understand the form. Developing this knowledge of your subject will help you create more convincing form whether you are rendering your subject realistically or more expressively.
I sincerely recommend you give it a try. Expect to make mistakes. Erase the lines that didn't work the way you expected and try again. It is more about learning and understanding the form than the finished drawing.