Saint Sebastian, a 3rd Century Martyr who was popularized in the middle ages as the patron of plague victims (and other epidemics), was allegedly the victim of such a "firing squad" of archers. So yes, it was indeed a used method, and you should be able to find a number of images from his martyrdom.

But the Romans were certainly not short of methods of how to put someone to death (there's literally dozens of rather descriptive Latin verbs for methods of killing). Infamously it was 6000 of rebellious gladiators and slaves around Spartacus who were crucified.

"The body of Spartacus was not found. A large number of his men fled from the battle-field to the mountains and Crassus followed them thither. They divided themselves in four parts, and continued to fight until they all perished except 6000, who were captured and crucified along the whole road from Capua to Rome." Appian, Civil War, 1.120

As for the command: It is indeed always weird when movies don't consider that. The commands "Notch!" and "Loose!" would be the more accurate (albeit I cannot point to sources that indicate this being authentic)