Why did Peter become a different person in Acts compared with before the crucifixion?
Peter was transformed because he witnessed the reality and power of Christ's resurrection, not a figment of imagination but real. Before the crucifixion he was fearful, scattered, and denied Christ three times to save his life. After the Resurrection, that fear was replaced by readiness to face persecution, knowing the Resurrection's power remained in their future.
Les teaches that Peter before the crucifixion was like the other disciples: when Christ was arrested, they were scattered like quail and scared to death. Peter denied Him three times because he was fearful for his own life. That was typical. But after the Resurrection, Les explains, you do not see that kind of attitude in Peter and the rest of the disciples. They are ready to go to the very ends of persecution, knowing that no matter what happens to them physically, the power of the Resurrection is still in our future. Les says to always remember that Peter, especially going into the Book of Acts, is a totally different person than he was leading up to the crucifixion. Why? Because he had witnessed the power of Christ's resurrection. It was not a figment of somebody's imagination; it was real. Les also points to Luke 24 to show how real and that He arose from the dead bodily.