Question: What lesser-known actors have completely stolen scenes from A-list stars?
Define “lesser known”.
A young Martin Landau stole a number of scenes from Cary Grant and James Mason in the classic North by Northwest

A young Angela Landsbury stole the film Gaslight from Charles Boyer and Ingrid Bergman:
A then-relatively unknown Richard Pryor stole a number of scenes in the 1974 biographical film Lady Sings the Blues:
A then-relatively new to films Jack Lemmon stood out in the Henry Fonda led Mister Roberts:
Despite being a vehicle for up and coming A-lister Johnny Depp, Leonardo DiCaprio steals every scene he’s in when he co-starred in What’s Eating Gilbert Grape?