Do you think that when McGonagall really got tired of her colleagues' shit she just turned into a cat and started doing random cat things? Like:
Dumbledore: Minerva, please, I really have to send those letters
McGonagall: *gets more comfy over Dumbledore's desk*
Dumbledore: Minerva
McGonagall: *starts chewing on a nearby quill*
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Umbridge: why are the cushions in my office so ruined? Who is responsible for
this?
Every other teacher: *looks at Mcgonagall*
McGonagall: *cleans her nails*
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Snape: Minerva get out of that jar now I need it.
Snape: You are far too old for such antics.
Snape: I'll stop taking points from Longbottom for a week, is that enough?
McGonagall: *flicks her ear*
Snape: How did you even get inside there...?
McGonagall:*meows and burrows deeper in the jar*