Stephen is accused of blasphemy against the Law and the Temple, he defends these accusations by calling his accusers to surrender their heart to the new work that God is doing through the resurrection.
The Apostles Peter and John stare in the face of the men that ultimately had Jesus killed and stick to their story, knowing full well that they were risking their own lives. After encountering the resureciton there is no alternative: proclaim what you've seen.
Acts isn't an apologetic, it's not meant to defend anything. It tells its story with confidence that it matters; no hint of ego or self consciousness. Acts doesn't spend a lot of time being theoretical, it gets right to living. And so should we.