AUSTIN – Sarah Palin must know her facts. Joe Biden must know when to stop.

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In the most anticipated vice presidential debate in decades, both candidates have something to prove to voters, who are looking for competent answers and something as ethereal as a look or a tone – some kind of connection to their lives, experts believe.

Because what survives in a debate are one-liners and impressions, and that is why the only meeting between Ms. Palin and Mr. Biden Thursday at Washington University in St. Louis could be so treacherous for both.

Ms. Palin, unknown to much of the country before her selection by Republican John McCain, has had several weak performances in interviews. That's led some – including a few high-profile conservative columnists – to question her credentials for the job. But don't discount the Barracuda: She shined in 23 debates in her victorious mush to become Alaska's governor, and is a master at winning over Joe Six-Packs, as she calls average Americans.

"She's very blessed. There's no question a lot of politicians would love to have her ability to connect with people," said Andrew Halcro, a former GOP Alaska House member and a 2006 independent candidate for governor who debated with Ms. Palin more than two dozen times.

But she spoke in vague "let's-shake-up-government" sound bites that are likely to be challenged in a national debate, he said.

"From an opponent's standpoint, it was frustrating. There was not depth," Mr. Halcro said. "It was all about public relations. It was never about public policy."

Mr. Biden, who went through 14 Democratic primary debates this year and last, exudes experience. But he can come off as a blowhard – an especially dislikable trait when juxtaposed with a charming woman. The gender sensibilities still resonate, and he cannot afford to either patronize or browbeat.

"Don't be too aggressive, and don't cut her off," said James Roland, a debate expert at Emory University.

If so, many will see him as offending her and the small-town everyman she purports to represent.

"He risks being seen as hot-headed, rude and insensitive," Mr. Roland said. "He has to be conscious of both his tone and his words."

Ms. Palin should acknowledge her awkward answers in recent interviews, perhaps with a quick, self-deprecating quip. "Then the more she can show she has an organized mind, structured thoughts and precise ideas, that will lend credibility," Mr. Roland said.

Public speaking consultant Ruth Sherman said Mr. Biden must show he's likable, even if he's not in the same league as Ms. Palin, and she has to show she's competent.

"It wouldn't surprise me for her to do very well," Ms. Sherman said. "She's a masterful communicator."

The outdoorsy mom and former beauty queen has appeal, and voters pull for people they like.

While she must show some command of the topics, "she can also project confidence, and that could very well substitute for the knowledge she doesn't have," said Ms. Sherman, author of Get Them To See It Your Way, Right Away.

Advice for Mr. Biden includes referring to his opponent not by her first name but as "Governor Palin," aiming his barbs at John McCain, and striking a conversational tone that is not condescending or preachy, she said.

"He shouldn't smirk or smile at her expense," Ms. Sherman said. "I expect she'll have some zingers for him, and he needs to be careful how he responds."

Mr. Biden himself expects as much. Last month in Chicago he surmised: "She's going to take a lot of straight lefts and jabs at me, she's going to try to get me to respond; she's going to try to get me to respond in a personal way."

She also has alluded to her posture in an interview with CBS. "He's got a tremendous amount of experience and, you know, I'm the new energy, the new face, the new ideas," she said.

University of Texas media professor Robert Jensen said that all puts Mr. Biden in an interesting dilemma, trying to show his opponent as ill-informed when many others see her as an average American with whom they identify.

"If he attacks Palin for her relative lack of knowledge and experience, he looks like he's beating up on an ordinary person trying to do her best. And he risks alienating female Hillary Clinton voters who are still sensitive about how their candidate was treated," Dr. Jensen pointed out.

"If he goes over her head and talks about John McCain, he looks like he's not taking her seriously," he said.

Mr. Biden can be long-winded and is "not known for being so good on his feet." Meanwhile, Ms. Palin has become the fodder of late night comedy, Dr. Jensen said.

"So, if I were an adviser to Sarah Palin, I'd be very nervous going into this debate," he said. "And if I were an adviser to Joe Biden, I'd be very nervous going into this debate."