This is my daughter, not exact, but an essence of her. The tell tale signs, hair, coloring, posture etc. If you aim for those little "extra" things that make a person an individual you will get close to that essence you're looking for. You get the essence, and you've captured something. It could be so simple, maybe the freckles, or hair that are dead giveaways. Look for something unique to that person, it might even be a larger nose as with the case of my husband, (see below) sorry love....but true. Those little defining twists are what make a person unique into themselves.
I used my Lamy Safari fine point pen, I really love that pen. And then added some color with my mini Koi watercolor set. Love this little travel set, you can take it anywhere.
YOU are the one that notices differences because you are really looking at them. Once you get to know someone you don't scrutinize their faces like you do a stranger. So I think the key is if you want to become a better sketcher....scrutinize everything! Really look at things , pull them apart with your eyes, and look for the essence.