The differences between facial expressions indicating shock versus those of surprise or fatigue are small.
In this article I will discuss my take on how you can tell these three emotions apart from the shape of the open mouth, along with other cues that point to a specific emotion.
When a person is experiencing shock, the mouth goes wide open, as in the accompanying picture. The mouth is open and makes the shape of the letter “O.” The eyes are generally wide open to the fullest extent and the eyebrows and forehead are pulled up as much as is humanly possible.
This is the classic look of a person who is in shock. I believe there is a difference between a shocked facial expression and one of a person who is surprised. Often a surprise is something that is happy to the person, so I would look for more of a smile while still having the mouth full open.
The second picture conveys the emotion of surprise better than the first one, at least in my mind. Her mouth is open, but there is definite smile involved.
Notice that the person is showing her teeth whereas the person in shock will tend to not show teeth. Of course, the surprise could be something negative, but that happens in a minority of cases.
With a negative surprise, there would still be an open mouth, but the expression would resemble more of a frown. That is actually pretty rare.
If you look up pictures for the emotion of surprise, you will see that nearly all of them are showing a smile, and the majority of them have hands to the face in some way: often holding a cheek or even both cheeks.
In the case of fatigue, you also see a wide open mouth, but with a yawn the hand is usually attempting to cover the mouth and the eyes are shut tight, whereas with surprise or shock the eyes are fully open.
A yawn can originate in different ways. Often it is a form of mirroring the gestures of others.
I am sure we have all caught ourselves yawning immediately after another person has done the same thing.
Another cause for a yawn is insecurity or doubt. If we are anxious about something, we will tend to yawn a lot more. Notice yourself yawning while sitting in the waiting room at the dentist.
With all three of these gestures, the mouth is wide open, but the ancilliary cues give us enough information to interpret the emotion correctly.
What is of interest here is that you need to assemble various bits of data in real time and put together a mosaic of the cluster of signals to interpret an expression accurately.
Several different emotions involve an open mouth, so you need more data than just that fact to understand what the person is experiencing.
The last statement holds true for all types of body language gestures. The particular one in this article is a case in point how slight differences can mean entirely different things, and you need to be alert to look at the whole picture.
There are two ways you can use this information professionally. First, you can ask the right questions based on an accurate reading of the other person’s emotions.
For example, you might ask, “Why do you find that statement to be shocking?” Alternatively, if you see a smile in connection with a wide open mouth, you might ask “What about what I just said is surprising to you?”
A second way you can use this information is to make note of your own body language in specific circumstances. Are you confusing other people when you yawn as opposed to reacting with surprise?
In other words, keep track of how accurately you convey your true emotions with your gestures.
In every case, you need to use Emotional Intelligence to make an appropriate reflection of how you are interpreting the gestures. Doing that will enhance the trust other people put in you and thereby strengthen your relationships.
This is a part in a series of articles on “Body Language” by Bob Whipple “The Trust Ambassador.”
Posted by trustambassador 
Watching how people deal with their jaw and chin can help you understand what they may be thinking. A good example is to watch for clenching of teeth.
When you communicate with other people verbally, in writing, or even in emails and texts, be particular about how you phrase things to draw upon the imagination of the receiver.
The topic of facial expressions is endlessly fascinating. Keeping in mind that all body language is culture specific; still many of the facial expressions are the same no matter what culture is employing them. For example, a child in pain is going to have the same facial expression regardless of where in the world he or she originated.