Drawing surprised faces won't be a surprise (bad pun, I know) if you have already learned how to draw fear. All the elements associated with fear is there except for the differences in the mouth.
As a refresher, the eyebrows are stretched upwards by the forehead. In doing so, the face is opening up the eyes as far as possible.
For the most part, the upper part stays the same as the fear expression. It's the lower part of the face that we will have to work hard on distinguishing.
Having said that, let's start drawing with your digital art tools to learn how to give your characters that shocked expression.
Drawing surprised faces in the front view is important to know what facial features to focus on. Since it's based on the fear expression, a lot of it will be very similar as stated before. If you have a copy of the drawing that you did in the fear lesson, use it as a reference point.
If not, here are a few things to focus on.

First, the eyes need to be made as wide as possible. Draw some wrinkles in the forehead to show the movement of the muscles upwards. Draw the eyebrows to correspond to this movement as well. At the same time, draw a few wrinkles between the eyebrows to show a bit of compression.
As for the mouth, this is where a lot of it changes. You want to make the mouth open wide as possible for now. For that reason, you need to make both the upper and lower lips compact itself towards the center as it opens up.
Spread the chins downwards as well. In doing so, the cheeks will relax downwards. Show a bit of the top teeth but not too much because the relaxed mouth will cover it.
Drawing surprised faces in the partial side view is, more or less, the same as the frontal view. The difference is focusing on the mouth so that it is not a perfect circle.
Try to imagine the curvature of the face, protrusions and indentations, as you draw the mouth.

You will notice that it will not resemble a perfect circle at all. In this view, try not to focus on the cheeks as much. You will notice that the cheek stops stretching near the middle of the face instead of stretching down towards the chin.
They eyes will require a bit of compression like all the lessons we have done thus far. You don't want it to be a perfect almond shape.
Don't forget to make sure that the pupils touch the bottom of the eyelids so it can reveal the 'white' portion of the top eyes.
The profile view is where it gets interesting for drawing surprised faces. Here, we still have the same concept as the fear expressions when it comes to drawing the upper part of the face.
The changes come from the bottom of the face. You will need to extend the chin of your characters (if you haven't done so already in all the examples) downwards. The mouth becomes half a circle where you need to draw protruding lips from.
Where all the expressions hid the corner of the lips by the cheeks, the surprised look has the corner of the mouths showing since the cheeks stop stretching near the middle of the face.

Hence, you might want to smooth out the color gradients from the corner of the mouth into the cheeks while, at the same time, making a distinction between the ending of the mouth and the cheek lines.
You can start working on other variations of surprises once you finish our primary example. What two extremes we can focus on is the expression of shock and the expression of being startled.
Depending on how you want to do shocked expression, you can play around with the eyebrows and mouth. Just make sure to keep the eyes as opened as much as possible. For the eyebrows, you can have one brow slanting downwards. This creates an expression of shock mixed with an element of disbelief.
On the other end of the spectrum, we have an expression that focuses more on the mouth than the eyes. We all have different reactions to being startled. For some, their months remain closed while others create a smaller opening.

In either expressions, you will notice that the cheeks remain neutral and do not stretch towards the chin.
As long as you keep these tips and tricks on the back of your mind, you can make many different and subtle variations of surprised faces as you like (or as the job dictates)!
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