Bite your nails, bury your face in your hands - do anything but react as the excitement heats up.
Crowds at the Rio Paralympics delight in making noise but when it comes to blind five-a-side football, the whole point is to stay silent so that players can track the ball by sound.
That can be hard, especially when it's Brazil playing on home soil and in front of crowds more used to the usual 11-a-side, able-bodied variety of football - and its screaming fans.
"Brazil, Brazil!" they chanted around the packed arena just before a game started against Turkey on Sunday. From the noise and the blaze of yellow Brazilian team shirts, it seemed unlikely these fans would be the quiet types.
But front row spectators Wagner Goulart, 30, and Luciana Vargas, 34 - decked out in full Brazil fandom gear and armed with a trumpet and a maraca - promised to behave.
"We only get going when the game stops or when there's a goal, of course," Goulart said.
And when the whistle blew, the only sound coming from the crowd was a group "shhh."
Paralympic 5-a-side is a version of the world's most popular sport for the blind or visually impaired.
To even things, all the players wear eye shades so that none can see anything. It's the sound of the ball, which contains bells, that they follow, as if seeing via their ears.
For 50 minutes the battle rages, with only the two goalkeepers allowed to see and to act as a guide. Clearly any noise from the crowd could ruin a play or even the match.
But Brazil's 2-0 win, securing a place in the semi-finals, was a tough test for Katia Brum, 38.
She watched Brazil's star, Ricardinho, move from the left corner to shoot and felt herself tense up. She put her hand to her mouth, desperately restraining the urge to yell and encourage and advise the player.
"It's too hard. It's the first time we've come to see a game with the blind and emotions run high. I could never bring my dad - he'd have a heart attack," she said. "We're used to helping, shouting, being that 12th man. I'm going to crack up."
For the players, the struggle in the stands has mixed blessings. They require silence, but love the energy.
"It's very important. People get motivated. It gives us extra gas. It may disturb us but we know it's important," said striker Nonato.
"I hope it continues like that," Ricardinho said. "I hope it's packed and that the fans can celebrate another title with us."
Paralympic 5-a-side made its debut at the Athens 2004 Games and Brazil has cleaned up on the gold medals ever since.
As on the regular football pitch, Brazil's big rivals are Argentina, which lost to Brazil in 2004 and won bronze in 2008. This year they are favorites to reach the final for another showdown.
Argentina's captain Silvio Velo, known as the Messi of 5-a-side, sympathizes with the crowd's noise-control issue.
"It's wonderful to play with fans, although sometimes it can work against us, because we know that football brings out a lot of emotion. But how are you meant to control that?" he asked after another game Sunday.
"When you're in a game and people start making noise, this could lose you the match. At the same time it could be your motivation," he said.
Expect some quiet - and very nervous - Brazilians if that final against Argentina takes place.