Although an initial look might make it seem as if acting and writing are two vastly different arts there are tools in an actor’s kit writers can borrow. Several of those come in handy for developing characters. For today, I’m only focusing on character description.
For an actor, costumes and makeup are only part of what they can count on to help change their appearance. Another item they might be taught in, is the leading center. In other words: what part of the body pulls a character forward? This goes a little deeper than just thinking about which foot to put forward first, it also helps define a character based on where their center of gravity is. Both a heavily pregnant woman and a rotund man will lead with their stomachs, leaning back slightly to offset the added weight at the front of their frames. The difference here however, is how that weight is carried: pregnancies tend to have supported weight, resulting in the waddle of an expectant mother. Excess weight from too many beers results in a shuffle as that weight drags on the body.
There are other ways of expressing this too. The daydreaming child leads with her heart, both literally and figuratively. The headstrong warrior puts their brave face forward first. An old man with a heavy weight lets his shoulders push him forward. Each of these people has a reason for the way they move: be that skipping joyously all over the place, or sidling along with a cane.
For writers, thinking about what would lead a character forward gives you a good base for their outward description that goes beyond short, skinny, brunette, blue eyes, etc. This gives you a way to describe their actions and fix them in your readers mind: how they walk and how they stand.
Not only that, it also gives you opportunities to work description in besides mirror scenes or self-comparison. Describing a character striding into the room to throw papers on his desk becomes a memorable opening. Similarly as a quieter character ducks their head and tries to make their stature smaller in a crowded elevator stands out more than someone looking in the mirror.
As an exercise: Observe people around you and how they move and stand. What part of their body sticks out most when they’re standing? How does their leading center affect how they walk?