Roman courts, civil or criminal, neither had the ability to coerce defendants to come to trial nor, had they had the capability, would they have typically been particularly interested in doing so. Imprisonment for crimes is a very recent phenomenon, really a concept that doesn't appear in the west as a regular thing until the nineteenth century. Rather, the penalty for conviction of criminal charges in the Roman world was pretty much death, over and out. Or, more precisely, the penalty was exile. By law there was a period of about three weeks, the trinundinum (lit. three market days), between conviction in court and the execution of sentence. Imprisonment during this period happened, but the typical thing to do was for the convicted party to flee from the city and go into exile, the technical legal word for which in Latin is simply fuga, flight. If this occurred--and there are basically no cases in which it didn't--the people passed in the assembly an interdictio prohibiting the condemned from receiving fire and water, the legal formula for exile and loss of citizenship. In the Principate the status of exile was more complicated, but the basic fact that exile was technically self-imposed flight from justice, and that condemned men were permitted to flee justice, more or less holds throughout Roman history. Similarly, rarely were any serious measures taken to prevent someone from skipping town before his trial, though it was not common.

Why? The fact that there was no police force is not necessarily important. I said that imprisonment before execution of sentence sometimes occurred, but such cases were limited pretty much exclusively for people convicted of crimes against the state, who were considered too dangerous to be allowed to go into exile. This is important. Roman citizens, after the end of the conflict of the order in the early third century, were protected against coercion, and could not be executed without the approval of the people. At least that's what even scholars often say, since the legal categories are a little more complicated. More accurately, a citizen who appealed to the people or to the tribunes (at a certain point the distinction became meaningless procedurally) and whose appeal was accepted could not legally be coerced or put to death. This was a fundamental right of Roman citizenship, the fundamental right in the view of "constitutional" scholars going back to Mommsen's Staatsrecht. It was a right that did not necessarily apply to criminals against the state. No appeal was possible for those declared hostes by SCU, and it was never explicitly invoked by criminals against the state except once, when the tribunes for ten years straight refused to acknowledge the appeal.

It was therefore fundamental, as a central cultural custom, for citizens in all but the most extreme cases that the condemned be allowed the opportunity to escape, avoiding death but assuming disgrace and the loss of all rights. Flight prior to trial happened, but only extremely rarely--if the defendant did not show up at his trial he could still, by Roman law, be condemned in his absence, which was considered an admission of guilt. In very rare cases the accused were imprisoned, with the intention of preventing them from fleeing. In 63 during the Catilinarian conspiracy the so-called "urban conspirators" were arrested by Cicero, who claimed that the authority of an SCU passed weeks earlier gave him the power to arrest these conspirators as well, a legal point contested at the end of the year by some of the tribunes. Similar cases were not entirely unknown, although it's important to keep in mind that the conspirators had not yet been formally accused of a crime and that their imprisonment was justified not by suit but by an SC. In such cases, as in 63, the accused would be held in the homes of prominent citizens, usually senators but sometimes equestrians or even sitting magistrates, to prevent them from escaping and, under normal circumstances but not in the case of the Catilinarians, to ensure that they showed up to their trials. So, Lentulus was put in the house of Lentulus Spinther (aedile 63), Cethegus in Q. Cornificius', Statilius in Caesar's, Gabinius in Crassus', and Caeparius in the home of a certain Cn. Terentius. From there the Catilinarians were taken to the Tullianum and executed without trial.