I agree, Jamaica, James Cagney developed wonderfully from his very earliest movie in 1930 (pre-"Public Enemy" he played either yappy, spoiled, criminal Momma's boys or loud salesmen - his first films being 1930's "Sinner's Holiday", an atmospheric story about carnival people based on the 1920's play Penny Arcade - and then a bit part in the achingly dull George Arliss picture "The Millionaire" -- but in the 1931 "Doorway to Hell" as second lead to Lew Ayres, his character really shone and he actually should have bene the leading man. Still, there was that loud, fast, excitable and overwrought style of 1930's acting -- admittedly it's a dated technique and often doesn't sit well with modern audiences/today's viewers.
Then when one sees an older Cagney, he's mellowed out slightly, although there's still that vaguely histrionic style of the 1940's-50's - (that style among all actors of the day carried over into 1960's television -- look at those actors in The Twilight Zone series - some of the performances even in the 1960's were downright hammy.
But then, as a mellow elderly man - so soft spoken and artisically sensitive in interviews -Cagney's "realness" comes out, and it is a very fine-honed presentation. I'm a big Cagney fan from the get-go.