Im surprised no one has yet mentioned the Dunning Kruger effect. To save you the trouble here is an excerpt from wikipedia:

"The Dunning–Kruger effect is a cognitive bias in which an unskilled person makes poor decisions and reaches erroneous conclusions, but their incompetence denies them the metacognitive ability to realize their mistakes.[1] The unskilled therefore suffer from illusory superiority, rating their own ability as above average, much higher than it actually is, while the highly skilled underrate their abilities, suffering from illusory inferiority. This leads to the perverse situation in which less competent people rate their own ability higher than more competent people. It also explains why actual competence may weaken self-confidence: because competent individuals falsely assume that others have an equivalent understanding"

You cant judge a person's competence from their level of confidence.

As for how people react under pressure, this is reasonably stable personality trait. Stoic people tend to be stoic most of the time, highly reactive people tend to be highly reactive. Of course even the coolest person can blow up if she has had a tough week, no sleep, and too many fools to deal with.