Robert Downey, Jr. was recently a guest of Jimmy Fallon. In the video above Tony Stark runs a mini audition for his Tonight Show host. While Fallon is much more of a comedian, he certainly can act.
Although there is some laughter suppression occurring here, this is a pretty fair job of the classic "slack jaw" of emotional shock (wherein the emotional mind is trying to process what the intellect has just discovered). The jaw is only partially opened during emotional shock - although we wouldn't typically see even a trace of exposed teeth. His eyes are opened slightly wider than baseline here as well - with a moderate level of tension in the lower lids and mind-face. Again - this expression is a fairly good job a replicating the emotion cluster of shock-transitioning-to-fear.
Specifically Jimmy's eyebrows elevate here - yet his eyelids partially close. During sincere surprise (which is very short-lived and rapidly replaced by another emotion) the eyelids open wide - they NEVER close as we see here.
Of course we know Fallon is acting - yet during feigned surprise (which, if you know what to look for, you'll see every day all around you) it is meant to deceive you - thus it's a form of acting as well (only then we don't want to have our disbelief suspended).
Fallon also misses the mark with his mouth. The mouth of significant surprise (unlike that of emotional shock) is a widely opened one - in the shape of a vertical oval. When the surprise is sincere, it also very characteristically does not display any visible teeth. This is absolutely NOT the face of surprise.
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