Andrea Martin is least interesting when she is being herself, but fortunately, on television anyway, she rarely is. A gallery of her old, funny, knockabout SCTV characters comes merrily back to life tonight in "Andrea Martin: Together Again," at 10:30 on the Showtime pay-cable channel. Once more, for example, Martin dons the too-tight leopard-patterned suit of unsinkable Edith Prickley, the earthy-mirthy dynamo who, we discover tonight, has fallen from the position of network programmer to that of bartender. Or is that rising? From behind her bar, Edith -- so old she can remember "when the Dead Sea was only sick" -- dispenses such Prickley pearls as, "Well, whatever gets you through the night. For me, it's a package of beer nuts and a rerun of 'Lassie.' " The script, which Martin co-wrote with Mike Short, is not nearly so bright and devilish as "SCTV Comedy Network" was on a good night, and there were many. But it has its moments, some of them shamelessly physical, like Martin steadfastly refusing to be pried out of bed by her spiritual guru (Martin Short), or, as the sensitive harpist Yolanda, suffering harplash from a string that breaks and boings in her face. Self-helpfulness in many of its most cloying manifestations is what's mainly being spoofed. The premise is that Martin has come crawling back from one of those hellish abysses that celebrities are always crawling back from these days, and is starring in a TV special called "A Celebration of Me." Of course virtually everything that could go wrong does. The studio audience for the torturous affair includes Martin's former inmates from the Institute of Bold Movement, a gaggle of gawking group-therapy groupies, and her perfectly ghastly family, worse than any ever conjured by Eugene O'Neill. Hooting and heckling from among the tables that surround the stage, her boorish father demands a margarita. To shut him up she orders him one, and then wouldn't you know, everybody wants a drink. Soon they are all whirring Waring blenders at their tables and ignoring Martin's heartfelt pleas for love and understanding. In addition to Short, an SCTV alumnus himself, former Martin ally Dave Thomas drops by, playing Ted Boil to Martin's Edna. They still own the organ emporium but have branched out into the seedy-hotel business. And the buoyant, beaming Catherine O'Hara appears as Dee Lee, a caring and supportive feminist filmmaker whose latest work is an unctuous "Best Friends." During this film-within-a-show, Martin reprises the character of Dutch, the brawny pool-playing gal whose broad shoulders keep threatening to swallow up her head. O'Hara, for her part, can still induce giddiness better than anything but helium. Some of the gags go too far (Edith Prickley flashes back to her stint as JFK's mistress), but the freedom to go too far is one of cable TV's few assets. More distracting is a very bad soundtrack mix that, at least on the tape submitted for preview, blurred some of the dialogue. Unlike Short's recent HBO special, though, Martin's is neither pretentious nor self-glorifying. Indeed, she subjects herself and her characters to so many mortifying indignities that the humor can get awfully cruel -- the old, raw SCTV irreverence, still capable of keeping a viewer breathlessly off balance. One other thing: The hour flies by. Before you have time to tire of whoever Martin is at the moment, she's somebody else entirely. And often, entirely hilarious.