The script of the Unicorn Theatre's new play uncannily echoes a quarrel in the star's dressing room. And the stage drama gets all too real when charming Felix Gardener shoots his blustering rival, Arthur Surbonardier, dead-with a gun Arthur himself loaded with blanks. Or did he? How the live bullets got there, and why, make for a convoluted case that pits Inspector Roderick Alleyn against someone who rates an Oscar for a murderously clever performance