Major Roman sources including Seneca the Younger, Apuleius, and Petronius refer to the damnatio ad bestias in which condemned persons were placed in an arena with predatory animals, as part of a public spectacle in front of crowds. These spectacles could include exhibitions of imported animals, theatrical hunts, artistic installations, gladiatorial games, and other entertainments that attracted large crowds, with executions being only part of the programme. The Code of Justinian, which set down earlier Roman laws, includes specifications of which crimes could result in which types of execution, including the damnatio ad bestias.

In The Colosseum, Mary Beard and Keith Hopkins mention that events at the great Colosseum in Rome were free, though an invitation or ticket was required. These might be obtained through patronage or other informal networks. The Colosseum could hold over 50,000 people and the ancient writers noted above refer to it being well filled. Other cities had smaller amphitheatres used for similar purposes.

This open access book chapter by Margherita Carucci addresses the role of spectacle in Roman justice, and may be of interest to you.

ETA The population of Rome during antiquity is estimated at 450,000, so more than ten percent of the population could attend a given spectacle. So it’s likely that ‘average’ Romans, for whatever definition you care to specify, did indeed attend executions.

Further ETA that beheading was a common execution method for Roman citizens. Noncitizens, including slaves and your hypothetical POW, would be subject to different punishments more amenable to spectacle. See the Code of Justinian, linked above.