In a play or film, the people in charge “cast” the show: they choose which actor plays which part. If they miscast someone, they gave the part to an actor who does not fit it — too young, too light, the wrong kind of energy. You can say “She was miscast as the villain,” or that the whole film was badly miscast. People stretch it beyond the stage: “He is miscast as a manager” means the job does not sit on him. The past and the -ed form are both miscast, not miscasted — same pattern as cast. It can also be a noun: “The lead was a miscast.”