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Hi Scom! Sorry for the late reply - Alder's Blood release is just around the corner so everyone's are super busy those days.
Every action in game use some amount of stamina. Moving, attacking or reloading. And when a character has run out of stamina - he does that 'crunching' animation. TL;DR - you need to manage your stamina carefully. 
As for the outline - moving within the outline doesn't cost stamina. Moving outside of it - does.