There is only one thing for it - if you can't beat 'em, join 'em. De Niro is dangerous. He doesn't ease up just because this is comedy. When Vitti puts a gun to Sobol's head, you know he is capable of pulling the trigger. As always, his performance has absolute conviction.
Crystal is subtle.
He lets the other guy lead, while working on his responses. He did this to great effect in Father's Day, with Robin Williams. He does it here, underplaying the farce, in order to milk the situation of its human fallibility. The plot doesn't add up to the sum of its parts. Perhaps with Mickey Blue Eyes and others, Mafia humour has been exposed as limited.