If you want to remember an interesting face in order to draw it later, first make yourself familiar with a variety of heads, eyes, noses, mouths, chins, cheeks, necks and shoulders. There are ten types of noses: straights, bulbous, hollow, prominent above or below the middle of the face, aquiline, regular, flat, round or pointed. These descriptions apply to the profile.
Looking full into the face, there are 11 types: equal, thick in the middle, thin in the middle, with the tip thick and the root narrow, or narrow at the tip and wide at the root, with nostrils wide or narrow, high or low, and the openings wide or hidden by the point.
You will find an equal variety in the other details of eyes, mouths, etc. Draw them by observing them and remember the characteristics in your mind. When you have to draw a face by heart, carry with you a notebook in which you have noted this variety of features and when you have seen the person you wish to draw, refer to your notebook and find the characteristics that match. Make note of these, and then later you can work on your drawing in private.