This is one of the fundamental rules of fiction, and it means showing your reader the action as it happens, not reporting it. Avoid long screeds where a character describes past events; instead, write him or her into the actual scene, complete with dialogue and descriptions. Rather than writing, "Marcel vented his rage on the bank teller ", you might insert a short, spicy scene demonstrating your character's lapse into fury :
" Marcel's arms started to sweat with irritation. Banks. He loathed everything about them: the grey office carpeting, the queues, the endless mess-ups.
'Yes?' the gum-chewing assistant sighed from behind the security panel.
Blimey, it was Dwayne, the one with the diamond earring, the same inept fellow who'd bungled his forex for the Canadian business trip. Suddenly, Marcel could not hold back any longer..." And so on. Let him say exactly how he feels.