Some jailers were public slaves. Prison directors (whether slave or free, as may be likelier here; cf. 16:33-34) could receive good pay.
Guards were often harsh with prisoners. Stocks were used for low-status prisoners not only to secure them but also for punishment and torture; legs could be locked into various painful positions. Apparently all the prisoners were confined to the “inner cell” overnight, which would suffer from overcrowding and poor ventilation.
The psalmist mentions being put in bonds (Ps 119:61), but then speaks of singing at midnight (Ps 119:62).
Earthquakes were common in this region, though they normally would not selectively target doors and chains while sparing people.
If faced with a dishonorable execution, Romans typically considered suicide the nobler way to die. The chief jailer may not have been held accountable for escapes in view of the earthquake, but in principle a guard who let prisoners escape through negligence could face severe consequences (cf. 12:19). Many Jews, however, considered it normally shameful (as people generally considered it under normal circumstances).
Prisoners may have remained because of the guards (implied in 16:29) or because Paul urged them to do so. Roman law treated escape from custody as a criminal act, but often treated with favor those who refused to escape.
Inner cells (16:24) were very dark; the jail official requests torches or perhaps lamps from his subordinates.
The jailer probably knows the charge (involving their Jewishness) and something of their message (about salvation, 16:17).
In Roman custom, the whole household would follow the religion of the head of the household, normally the worship of the respected Roman deities.
Prisoners normally were unable to wash or trim hair in jails. The jailer undoubtedly takes them out of the jail, which could have gotten him in severe trouble, especially if they tried to escape (16:23). Some suggest a fountain in the jail’s courtyard; since jails were usually in center cities, various public fountains are possible, although these increased the risk of being seen by Philippi’s night watchmen.
Jails and prisons typically provided only the barest sustenance, so that prisoners had to depend on outside help. The jailer takes a major risk: he could be severely punished for feeding and eating with a prisoner (in some cases death; in this case, certainly at least losing his job).
Sometimes public beatings, humiliation and a night in jail were considered sufficient punishment (though the earthquake may also play a factor here).