Acting in general. He’s not doing like Heath Ledger Joker stuff, but he’s been playing different characters with completely different personalities, convincingly, for the better part of a decade.
From a cute college freshman with a lifelong crush on his Phi, to a super flirty, rebellious student infiltrating the ranks of a super fucked up prep school to investigate his uncle’s suicide, to a alcoholic, coke using party kid in love with his best friend, starting a complicated friends-with-benefits situation with the leader of a band, to a professional hitman in need of normalcy.
That’s not even all of his roles, but they aren’t all the same character, they have their own quirks, issues, etc.
Plus his chemistry with his onscreen partner is legendary.