When describing the physical manifestations of sadness on a person's face, it's important to pay attention to the small details. For example, you could describe the way their eyes become red and swollen from crying, or how their mouth trembles as they try to hold back tears. You could also describe the way their shoulders slump or how they withdraw into themselves. By focusing on these small but telling details, you can create a more realistic and relatable portrayal of sadness.