To some extent, actors are at the mercy of the productions in which they appear. An exciting show can make an okay actor look terrific; an excellent actor can seem weak if trapped by circumstances. So far this season, Trina Magness has emerged with honor from two productions that simply didn't work. She played the first witch in the noisy, ill-conceived Macbeth staged by Listen Productions, giving the harridan a sinuous and insinuating grace that chilled the blood. And in Red Herring at the John Hand Theatre, while the play lacked finesse, Magness came close to redeeming the evening with her Maggie, the stereotypical detective's doll, a role she played with dry and understated wit.