What is a name or an adjective for a person who is easily influenced by the situations they are in and therefore exhibits intemperate behavior?

Interesting! That was me who put that forward on Ask! :) You wouldn't think so.

But I should add something: everyone who answered me there, had included words such as sheep, fickle, and follower. But these describe a person who can't decide for themselves or just want to evade responsibility. But what I had described, or think I had done so, was someone who, being asked to say their opinion on something, for example, would evince some grotesque gestures and behave as if they were an expert. Or having seen their friend in an actually simple quarrel, will beat up the other side, the 'enemy'!!  

Yes, impetuous, impulsive, and manic kind of communicate what I mean. Had completely forgotten about 'impulsive'.

Impulsive, impetuous, manic (of behavior), melodramatic best describe what I meant. Histrionic as well, but there seems to be an 'intention of getting attraction' implied. Others seem, to me, to communicate a particular aspect of the behavior I meant, and some seem to be more irrelevant than relevant, by having just one aspect in common with it.