Fear and surprise are notoriously challenging to apart at times. Here are a few tips for helping you identify the differences.
Surprise raises the eyebrows and lifts the eyelids (FACS Codes 1+2+5). Often there is the opening of the mouth. Surprise will also turn into another emotion as the situation is understood – in other words – is it a good surprise (such as a birthday party or visit from a relative)? A sad one (such someone dying)? A disgusting reaction to surprise (such as at first being surprised to finding an animal previously chewed on by the cat, followed by disgust)? Could the surprise turn into fear as we realise someone is in danger? Surprise is the initial reaction, until our brains work out the meaning of the surprise.
“Surprise is the briefest of all the emotions, lasting only a few seconds at most. In a moment surprise passes as we figure out what is happening, and then surprise merges into fear, amusement, relief, anger, disgust, and so forth, depending upon what it was that surprised us, or it may be followed by no emotion at all if we determine that the surprising event was of no consequence. It is rare to see a photograph of surprise. Because it is unexpected and the experience is brief, a photographer is rarely ready to shoot and even if he is he may not be fast enough to capture it once something surprising happens. Press photographs usually show reenacted or posed surprise.” (Ekman, P. 2003, p. 148 Emotions Revealed).
Fear is signified by (the FACS codes) 1+2+4+5, which is the raising of the complete eyebrows (1+2), then lowering of the brows (4) which creates a push / pull, and finally (5) raise eyelids. There could also be mouth movements (such as the lip stretch AU20). The major difference between fear and surprise is the brow lowering (AU4) which fights against the raised lids (AU5).