Mark 16:7-8 ends on a cliff-hanger: the tomb has been unsealed, the “youth” in white (i.e. an angel) has proclaimed to the women that the risen Jesus is waiting to reunite with the disciples in Galilee, and the women hurry off in reverent silence. At this point the curtain drops, leader the reader to wonder what would have happened next if Mark’s narrative had continued and what might be their role in the story in proclaiming the good news about the resurrected and vindicated Jesus.
The open ending of Acts 28:30-31 has a similar effect. Paul hinted at his impending death in his Miletus speech (20:24, 25, 38), was seized as a perceived agitator in Jerusalem (21:27-36), stood trial before the Jewish Sanhedrin (22:30-23:10) and the Roman procurators Antonius Felix (ca. 52-58 CE) and Porcius Festus (ca. 59-62 CE) alongside King Herod Agrippa II (ca. 53-93 CE) (23:33-24:27; 25:1-26:32) in echoes of Jesus’ passion, made a fateful appeal to the emperor (25:11-12; 26:32; cf. 23:11) that lead him to be transferred to Rome on a perilous sea journey (27:1-44), and was placed under house arrest in Rome (28:30). Readers may wonder about the outcome of Paul’s trial in Rome. Indeed, one of the classic arguments for dating Luke-Acts before 64 CE is that the book was finished before the outcome of Paul’s trial was known; the book also does not note the martyrdom of Peter or James (cf. Colin J. Hemer, The Book of Acts in the Setting of Hellenistic History [WUNT; Tübingen: Mohr Siebeck, 1989], 365, 407-8). There is a further hypothesis that Paul was initially released from Roman imprisonment, possibly explaining his further travels documented in the Pastoral Epistles and 1 Clement 5:6 (cf. Romans 15:24), before he was re-arrested in Rome and executed (2 Timothy 4:6, 16).
On the other hand, there seem to be subtle hints in the texts above that the author of Acts knew that Paul’s solitary Roman imprisonment and his testimony before the emperor would end in death. Moreover, Acts was not writing a biography of Paul, nor of Peter in the first half of the narrative, but explaining how the gospel spread from Jerusalem to the ends of the earth by reaching the heart of the Empire (1:8). Since the narrative achieved its purpose in the final act, the curtain is closed at this point.