"SHE DOES NOT KNOW WHO SHOT J.R. -- REALLY, SHE DOESN'T," blares over the loud speaker. The manager of the store where Mary Crosby is signing autographs is trying to dissuade the crowd that is fixing a mean stare on the wicked vixen known as Kristen on the hit TV show "Dallas." The odds are running 4 to 1 that Kristen shot television's greatest bad guy last season.

"I've been plugging her about it all morning myself, and I still don't know," the manager insists. Nobody buys it. Next.

"Tell me something, you didn't do it, did you?"

"I don't know," Crosby says impatiently, but she offers more than her usual pat answer. "Nobody will know until the show's fifth episode. But I would've loved to do it. Would do wonders for my career." Good enough. Next.

"Mary, do you still dance? I heard you studied classical dance for 13 years."

"Well, you either keep it up at that level or you do something else. I've always wanted to be a serious actress, and that's what I'm working hard at now. I'm leaving 'Dallas,' you know." She pauses and appears to ponder that last revelation. When the question pops up again, through, she answers straightforwardly, expounding on the reasons behind the move.

When "Dallas" finally begins its new season in November, Crosby says, she'll only be in the first five episodes but will return for a few guest appearances. "They need fresh blood," she says matter-of-factly. "The character can't go any further. She's slept with her brother-in-law, drove her sister to drink, become a blackmailer . . . What else can she do?"

Crosby's Kristen rivals only Larry Hagman's J.R. Ewing as the most dastardly character in "Dallas," the CBS prime-time series with viewers from no less than 57 nations wondering who plugged the man "America loves to hate." Kristen, the sister of J.R.'s wife Sue Ellen, is a likely suspect. She's television's All-American Bitch.

"Hi, Mary. Why, you seem so nice. I was such a fan of your father's, but I just know you did it. How can you play such a bitch?"

Crosby is wearing a billowing blouse tucked into tight jeans that outline her lithe figure. She plants her spike-heeled suede boots firmly on the floor, throws back her long brown hair and laughs. "My theory is if I get it all out at work, I can come home and be an angel. I'm not sure it always works, though."

Crosby was in town this weekend with her husband, songwriter Eb Lottimer. He watched as she signed autographs at Woodward & Lothrop. They are a handsome, wholesome looking couple, who live on a secluded hill in Malibu with "two cats, two dogs and two Arabian horses." Crosby's appearance at the store is one of the features of Woodies' centennial celebration. It's part of a job she took during the actors' strike, moldeling for the current Woodies catalogue advertising lavish table settings. m

But on Saturday nobody asked her about China and silverware. The questions were the same ones people have been asking her -- and every "Dallas" star -- since the show skyrocketed to the top of the ratings last year. And since joining the list of suspects in television's most celebrated event, the sultry daughter of the late crooner Bing Crosby admits that her sudden public appeal is divided largely into three aspects: the show, her father and herself, in that order.

Playing the power-hungry Kristen Shepherd, an outsider to the oil-rich Ewing dynasty, has been a "reach" for her, Crosby says.

"It took me a few shows before people would say, 'God, is she a bitch.' But it's been good for my career. A lot of people have seen me, and I've worked with good people. When you work with good people you learn a lot. And I'm here to learn," she says, her voice now filled with youthful ambition and her faced flushed with cherubic naivete.

Although Kristen Shepherd is an outsider to the Ewing family, Mary Crosby is a special member of another dynasty. Of Bing Crosby's seven children, she is his only daughter. "I loved being daddy's only little girl. But, oooh, he was so careful not to spoil me," Crosby insists.

An elderly woman, among a blitz of teen-agers and young mothers, takes her turn at the head of the line. "I was such a fan of your father's, Mary. It just tore me apart when he died."

"Oh, but you know he's so much happier where he is now, even though he lived a beautiful life," Crosby says, comfortingly. Later, she explains, "I said that because she was being sad about daddy's dying. I'm very good about his death. He was not senile, old, crippled. He had his voice." She stresses the latter. "I think he would've been very unhappy as an old man. He was never an old man. My God, he died on the golf course."

Crosby got her Equity card at 4, toured in summer stock with her mother (Bing Crosby's second wife, former actress Kathryn Grant) at 9 and appeared on all of her father's holiday speicals from '67 to '77. At 15 she entered San Franciso's American Conservatroy Theater.

Now 21, she played J.R. Ewing's lover and confidant for 25 episodes last season, finally becoming the classic " women scorned" in the last show the same night, by the way, that J.R. got his.

Carolyn Doekel, a young woman from Wheaton, Md., gets her turn in the autograph line minutes before Crosby's store appearance is over. "You're gonna leave the show?" she asks with surprise, confirming what she'd heard earlier in line. "Then you must have done it," she says pointedly.

"No, no, no," Crosby cautions. "I wouldn't jump to that conclusion." But Doekel says she is a "hard-core 'Dallas' fan and parks her purse for a long chat about the suspects.

But Crosby interrupts, "I'm glad you're enjoying the show. Now who do I write this to?"