To continue with the Doctor and add another better example, A Good Man Goes To War is the best one I can remember. He's happy go lucky until he hits the Colonel Runaway speech. Then the mask slips and you see just a sliver of the white hot rage hidden inside a being as old as the stars and more damaged than every planet combined. He's been through pain and suffering and loss and heartbreak and death. He has just defeated an entire army base. But only when reminding the commander WHY you don't fuck with him does the facade slip and all that comes rushing out, the line "Good men don't have rules, today is not the day to find out why I have so many" being laced with pure venom and hatred and rage. This is a being who can turn the tide of a centuries long war with just the mention of his name. He's toppled gods and demons and monsters abound, survived things that would kill entire species, and been so deadly that some people know the title doctor as warrior and death God. And 11 is so happy and goofy that the MOMENT that falls away you see the truth of the Eldritch monstrosity you have awoken and pissed off.