The purple line down this profile sketch shows how the profile lines up against a straight line.

See how the eye is not flush with the edge of the face. The eyes are set in—recessed. The front of the eye usually lines up with the outer edge of the mouth, more or less (The guy in this sketch has a narrow mouth and deep-set eyes, so it won't quite line up for him.)

Notice how the nose sticks out quite predominantly from the plane of the face. Also, the forehead slopes in just a little bit when it goes up to the hairline.

On this second illustration of the profile, notice how the lower lip doesn't come out quite as much as the upper lip. Notice that the chin and "muzzle" area (mouth and chin) recede, just a little. Even in this particular portrait (a man with a strong chin and jaw) the chin and mouth area do not jut out quite as much as you might think. They are gently receding in - just a little bit.

Occasionally, you'll see a person who has a chin that juts out more noticeably. That's the fascinating thing about faces - for every "rule", there's an exception. All the more reason to really look at your model, to see how their individual features line up.

Also, note that this face doesn't have what would be considered to be an especially large nose. And yet, the nose does stick out quite a bit. One of the mistakes of "newbies" (I remember doing this) is to make the profile too flat. The beginner portrait artist may tend to not make the nose stick out enough. It is like they made a straight line, and had the features stick out from this straight line. This makes the face look too flat, and quite odd.

The face is not set on a flat plane. It is more of an oval or egg shape. That's why the forehead slopes in a little to the hairline, and the chin recedes just a little bit.