Guests applaud First Lady Michelle Obama during President Obama's State of the Union address on Capitol Hill in Washington, Tuesday, Jan. 24, 2012. (Susan Walsh/AP)

As President Obama delivers his State of the Union address on Tuesday night, the first lady’s guests will be on their best behavior.

White House staffers will have coached those sitting in the gallery with Michelle Obama that at any moment the cameras might pan from the president’s podium to where they sit in the balcony. So they will watch their posture, stifle yawns and skip the chewing gum.

The everyday Americans invited to accompany the first lady as she watches her husband address Congress and the nation are essentially given roles by the White House. They are the human faces of the messages the president delivers, whether about the ingenuity of small business or the plight of returning troops.

This year, the president will focus on the economy and discuss issues such as gun control and immigration. A White House official said that victims of gun violence will be seated with the first lady, as will members of the middle class who would benefit from policy proposals that Obama will unveil, military families, and people working on immigration issues.

The speech will probably run an hour or so, and the next day, most guests will find themselves, Cinderella-like, back at their jobs and in schools and homes across the country.

After the president delivers the State of the Union address, there are rebuttals from either Democrats or Republicans, and sometimes other parties. Take a look back at some of the most memorable State of the Union responses of the last 10 years. (Nicki Demarco/Sandi Moynihan /The Washington Post)

Previous guests have found that the effects of the evening linger — in positive and sometimes tough or unexpected ways.

Attending the 2010 speech was a “game-changer” for Trevor Ya­ger. The founder of TrendyMinds, an Indianapolis-based advertising and public relations firm, Yager was invited, he says, to represent gay business owners thriving in a tough economy.

The attention he received — the local TV interviews, the national stories — caused a rift with a business partner. Soon after his return from Washington, the partnership dissolved, he said.

“There were jealousy issues,” Yager said. “When you have something like this come along, you do see people’s true colors.”

There were major benefits, too. He credits the exposure from attending the speech with the growth in his business. Trendy­Minds has grown from six or seven employees to 29. “All the coverage and attention helped attract clients and opened doors for us,” he said.

For Julia Frost, being a guest in 2010 complicated an already difficult relationship with her Republican father. The two are estranged, and although politics isn’t their only source of conflict, she says it contributes to the rift.

Growing up in a family of conservatives, Frost — who attended the speech as a veteran, a community college student and the wife of an active-duty Marine — says she’d always considered herself to be a Republican. But her visit to Washington made her question that.

She heard things from the president that she liked, about college and health care, and she was impressed by Obama’s demeanor.

“For a long time, I had been blindly following my family, but I saw that there was sense on both sides,” said Frost, who does not label herself a Democrat.

Another result of that night? She has a famous pen pal. Jill Biden, the wife of Vice President Biden, regularly corresponds with Frost. The women were seated together for the speech.

“She’s written three or four letters, in her own handwriting, and sometimes she’ll e-mail, just asking, ‘How are you? What are you doing?’ ” Frost said. “I was shocked that she’d do that.”

The transformative process for gallery guests starts on a clandestine note. A call from the first lady’s office a few weeks before the speech is the first step. Usually, the guests are people with whom the president or first lady has come into contact along the campaign trail or during an official visit. The prospective guest might have written a letter or e-mail to the president, or in the case business partners Kendra Baker and Zach Davis, posted a video on YouTube thanking Obama and members of Congress for passing the stimulus bill that afforded them a small-business loan.

The difficult part for potential guests is staying quiet; the White House usually asks them to keep the invitation to themselves — no media interviews, no big announcements on Facebook. The White House likes to make the announcement on its own terms, said Anita McBride, who was chief of staff to first lady Laura Bush.

Developing the list of invitees is a complicated process, she said. As the president’s speech is being drafted, staffers delve into databases to find people who could be the faces of various initiatives and themes. Then, as the speech is edited and refined, the first lady’s office and West Wing staffers produce a final list.

Staffers double-check that the potential guests don’t have any major skeletons in their closets that could be embarrassing.

“It was one of those times that the East Wing [shorthand for the first lady’s office] and the West Wing really worked together,” McBride said.

Hundreds of people have sat in that box since first lady Nancy Reagan invited Lenny Skutnik to join her in the gallery for the 1982 State of the Union address. Skutnik, a federal worker who plunged into the chilly Potomac River to save a survivor of an Air Florida flight that had crashed there, spawned the tradition of inviting everyday Americans to the speech.

Guests have included heroes such as Skutnik and Chesley “Sully” Sullenberger, the pilot who in 2009 emergency-landed a US Airways flight in the Hudson River with no casualties. But, of course, Americans whose achievements are less dramatic have also been invited.

“They represent, as average Americans, what the president is trying to say, and they make it meaningful,” saidMcBride, now a professor at American University.

That meaningfulness can flow both ways.

Zach Davis had just launched a gourmet ice cream enterprise in Santa Cruz, Calif., when he and Baker were invited to be guests of the first lady at the 2011 State of the Union. The media coverage helped accelerate the growth of the Penny Ice Creamery, he said. He and Baker have since added a cafe, opened a kiosk downtown and launched a second full-size location.

More importantly, he said, being chosen to represent small businesses and his home town fostered a sense of obligation to live up to the ideals he was supposed to represent.

“Ever since then, I’ve had this feeling of responsibility,” he said. “It’s like ‘Well, now we really can’t mess up.’ ”