The introduction makes it clear that this book will chronicle what happens after the disciples are baptized with the Holy Spirit according to the initial promise of Jesus. Perhaps it should be called the Acts of the Holy Spirit.
The final question the disciples ever asked to Jesus was a misguided one, "When will you restore the kingdom to Israel?" Jesus blew by it, "It's not for you to know. . . but you will be my witnesses."
The first prayer in the book of Acts starts, "You, Lord, know the hearts of all!" It changes everything when you realize God is looking into your heart the very moment you pray! They prayed to an omniscient God.
I'm still not sure if the choosing of the 12th person was necessary or not. Why did they feel the need to replace Judas? He was not, after all, a superstar. It would have been very weird to be Matthias!