After choosing between flank steak and chicken with prosciutto, the citizen-questioners will take their seats on the risers surrounding the stage and the tall swivel chairs that President Bush and Sen. John F. Kerry will occupy soon. They will have a chance to practice speaking into a microphone, but only the moderator will know what they would ask if given the chance.

Secrecy about the questions is one essential rule among many for what is expected to be the least predictable of the three presidential debates. Rather than a journalist designing questions, Friday night's town hall session will be turned over to the worries and musings of prospective voters.

The 100 to 150 participants were chosen by the Gallup polling organization. They may have opinions about the candidates. They may be leaning to one candidate or the other. But they must also have told a Gallup representative that they might still vote for the other guy. The campaigns call them "soft" supporters.

"We call them uncommitted voters," said Frank Newport, editor in chief of the Gallup Poll.

Gallup had sole discretion to choose the participants from a random sample in the St. Louis area. Working the telephone, the organization asked people whether they were registered voters, whether they planned to vote and whether they were committed to a candidate.

If they were undecided, they qualified, but "those are few and far between," Newport said in an interview. He pointed out that the most recent Gallup poll, published this week, showed zero percent of American voters undecided, the first time anyone can remember that happening.

If the Missourians answered that they favored one candidate, Newport said, they were then asked whether there was any chance they would vote for a different candidate. If they said there was a chance, Newport said, they were told they could join the town hall pool.

"Most of them," Newport said, "are very excited about doing it."

Preparations for the 90-minute session have been underway for months at Washington University in St. Louis, which was host for debates in two previous presidential years. The last piece of carpeting was laid and the final non-verbal nails were pounded in the school gymnasium Thursday as the campaigns and hundreds of journalists and technicians poured into town.

Just as the political protagonists were polishing their performances, the aspiring questioners were asked to do the same. Participants were invited to think up a question and be prepared to read it if the moderator, ABC anchor Charles Gibson, called on them.

"We just recruit the people," Newport said. "We have no idea what's the question they are interested in asking. The Commission on Presidential Debates has no idea, and neither do the campaigns."

Rules established during hard bargaining by the campaigns made it so. The 32-page agreement specifies that "the number of 'soft' Bush supporters shall equal the number of 'soft' Kerry supporters." It requires Gibson to choose an equal number of questions from each group and to make sure that he evenly divides the questions between two subject fields: the economy and domestic policy; and foreign policy and homeland security.

Only questions submitted to Gibson in advance will be permitted. There will be no follow-ups. In fact, the questioner's microphone will go dead once the question has been asked. Deviations will not be allowed.

Gibson has instructions to cut off a questioner who "poses a question or makes a statement that is in any material way different" from the original question. He will tell the audience that "non-reviewed questions" are banned.

The rules specify that Bush and Kerry will have "swivel chairs that can be locked in place." They will be the same height, and they will have backs and footrests. Each candidate will have a nearby table for water and writing.

Unlike in the other debates, the candidates may leave their chairs and carry their choice of a wireless hand-held or lapel microphone.

They may walk in their marked portion of the stage, according to the agreement. It states that "the predesignated areas of the candidates may not overlap."

Washington University is working to smooth the experience of the prospective questioners. The soft voters will assemble for a late lunch at a hotel and then dinner on campus. They will be asked to commit to steak or chicken, but may change their minds and choose other food from the buffet without losing their dining privileges. Gibson will not have a say in the matter.

Rocco Dattoli III puts finishing touches on the carpet in the gymnasium at Washington University in St. Louis.