To draw the human form the artist needs to differentiate between what is bone protrusion and muscular layer. Familiarity born of hard work is the only route. First, you need to draw the skeleton, and the scull if you wish to draw portraits.
How the eye sits in its socket and which bones protrude and which are hidden behind muscle and fat layers is more obvious when drawing old people than drawing the young. This is because the underlying structural components are more visible. As we age muscle and fat recede and the skeletal support revealed.
The eyeball is spherical and sits within a socket. The cheekbone ( the zygomatic bone) and the forehead bone ( the superciliary arch) dominate the local area. Surrounding the eye is an oval muscle: the orbicularis oculi, which compresses the eye closed.
To get started though I would position your sitter next to a window of natural light positioned above the model and with no other light source. Make sketches from many angles, try to deconstruct what you see into skeleton and muscle, surround yourself with anatomical reference from books, vary the model’s orientation to the light source and most important of all, think of everything in terms of geometric shapes. For example, the nose as triangular wedge shape, the eye as a spherical shape, etc.