In the theater, reality and make-believe blend so intimately that Tim Briscoe was convinced that he was playing the role of detective when he stumbled upon the lifeless form of Nellie Brant. But the corpse was real, even though everything and everybody else seemed fictitious.There was the elusive man who wore dark glasses, the actress who chose sudden death as the background for an audition, the ex-silent-film star who stooged on quiz shows for his daily bottle, and Maggie, who loved him but didn't believe in the effect of too many Scotches.This backstage mystery was written by a man who knew the theatrical world inside out. The characters and scenes are as authentic as Variety, as real as a closing notice.