The story was great, but the acting, except for the overacting by Cobb (which I think was purposeful to show how much pent-up anger he was carrying), was the part I love the most.
Especially considering 99% of the movie is just one room.
It was the amount of acting necessary to depict what was essentially an early caricature of a "deplorable", in accord with other films and literature of the era serving to undermine faith in the justice system, marginalize conservatives as angry, irrational, bigoted, etc. etc., and raise public outcry against capital punishment, even though we have voir dire to cull acceptable jurors.