Times Publishing Company   St. Petersburg Times April 24, 1992, Friday, City Edition SECTION: WEEKEND; On TV; TV PREVIEW; Pg. 16 LENGTH: 642 words HEADLINE: A sordid tale of sex and murder BYLINE: JANIS D. FROELICH BODY: Barbara Hershey says she took the role of small-town tramp Jimmie Sue Finger, who egged on her teen-age lover to kill her husband, because the real-life character was someone she had never even remotely played before. "Overtly sexual, platinum blond" is the way Hershey, who co-starred with Bette Midler in the syrupy Beaches, describes the seducer from Kennesaw, Ga.      Now she and Jane Alexander, paired in ABC's mini-series Stay the Night, deliver a salted peanut drama. It's a terrible tale about a teen-age boy's life gone down the tubes because of a vain, self-absorbed older woman. Who wants to devote four hours of viewing time to this? Well, the reason to stay with Stay the Night is that the Hershey/Alexander combo is so riveting.      Hershey and Alexander met TV critics last summer to talk about Stay the Night, but ABC hadn't scheduled the story until now. Alexander said a little; Hershey talked a lot. But both share center stage in this miniseries because the woman Alexander portrays, the teen's mother, displays grit and determination that make you cheer this wallflower as she takes on the town's Queen Bee.      Alexander plays Blanche Kettmann, a quiet and religious woman who is so initially naive, her own teen daughters have to explain to her what's going on between the 34-year-old Finger and her 17-year-old son, Mike (Morgan Weisser).      Her daughters bluntly inform her that a classmate's mother and Junior (Mike's nickname) are out together. "What are they doing?" asks Blanche.      Instead of lurid entertainment, Stay the Night does a conscientious job of showing how disgusting Finger's motives are, as she takes full advantage of Mike's raging teen hormones. He's lovesick; she's just sick.      The opening scene is especially revolting. There's mom at her daughter's party, dancing suggestively with the guy guests. So why doesn't someone tell Jimmie Sue to knock off the slut stuff? Where are the adults in Kennesaw, one wonders.      Finally, when her son is the target of Jimmie Sue's affections, Blanche summons up her courage to telephone Jimmy Sue to tell her to lay off. But the devious plot to knock off Finger's husband has already taken root.      Hershey explains that she chose not to meet the real Jimmie Sue. But in her research, she says that she found "lots of very vulnerable and sweet endearing qualities." Why so generous about a woman now serving life in prison? Hershey explains that her personal philosophy is that "nobody's all bad or all good."      Alexander did meet Jimmie Sue and offers some interesting insight about the strange friendship that developed between this straight-arrow woman and the sexpot who destroyed her son's life. "Blanche is an extremely compassionate woman," says Alexander. "She's always able to find the best in whomever. Jimmie Sue was a charismatic figure. And Blanche really, I think, felt her maternal instincts coming out with regard to Jimmie Sue."      The "friendship" between Blanche and Jimmie Sue had time to develop because while Mike was convicted of murdering Finger's husband, he refused to implicate his lover. Obviously, Blanche, as well as the police, knew that Jimmie Sue was manipulating her high school lover to kill her husband. But without Mike's cooperation they were powerless to prove it.       As Hershey points out, "Jimmie Sue to this day claims that she's totally innocent, that these were circumstances that just happened."      Hershey adds that she was intrigued by the 1986 murder case, because it's a study of seduction. "How does such a thing happen?" she asked. "In the process of Jimmie Sue seducing the mother as well, we get to follow and see how it can happen, how such a thing can happen with the boy and then the mother."