Anytime a comedian takes on a frantic role and nails it.

Off the top of my head….

Robin Williams: Dead Poets Society (and many more, I think that was his first) Bill Hader: Barry. Jim Carrey: Eternal Sunshine of the Spotless Mind

All these dudes are known for their acting though first and foremost…

They weren’t though, they were straight up comedians. After all these years it’s obvious they had more to them, but when they first stepped into serious roles it wasn’t known and was a surprise to most what they could do outwith comedy.

I hear you and agree, though for the sake of discussion and argument - let’s say OP was referring to unnamed nobodies who nailed it in their acting debut.

Who’s your best?