Bilal is correct in verifying Anthony's comment; one shooter is given a blank. This way, everyone in the squad can assume that they were the one with the blank. There is a psychological phenomenon called "diffusion of responsibility" that describes this feeling. Wikipedia article.
I also found this page about modern firing squad rituals from all around the world, including, to my shock and horror, Utah.
Here's a segment from a paragraph on Thailand.
On the day of execution, the prisoner was taken from their cell and photographed and fingerprinted. They were then taken to the execution chamber and handcuffed to a cross like wooden frame with their back to the machine gun, 4 meters behind them. A white cloth blindfold is applied and the hands tied with a sacred Buddhist cord. Flowers are hung from the prisoner’s hands as an offering to Buddha and a canvas screen is pulled between the condemned and the gun. A target is fixed onto the screen level with the prisoner’s heart and the gun aimed at the centre of the target. The executioner takes up his position, watching another member of the execution team who raises a red flag, and on the signal from the prison governor, the flag is dropped and the executioner fires a fully automatic burst of 15 rounds into the victim’s heart.
The picture is of Thailand's last shooting executioner. The country officially moved to lethal injection in 2003. An interview with him is available here.