In this chapter we see some significant changes in the church at Jerusalem. James has been killed and Peter is about to die at the hands of Herod Agrippa. It is only by a miraculous rescue that Peter avoids martyrdom in A.D. 44.
After Peter is rescued from prison, he tells the group which had been praying for him to report to James what has happened. This is James, the Lord’s brother. At this point in the story, we did not know that he was a believer, but he will be one of the major leaders of the Jerusalem church by Acts 15.
During Jesus’ ministry, the Lord’s family did not believe that he was the messiah. After the resurrection we are told in 1 Cor 15:3-5 that Jesus appeared to James at some point. We presume that after this appearance, James became a believer in Jesus as the Messiah. James has a reputation for being an extremely zealous Jewish believer, and a leader among the Pharisees and priests who accepted Jesus. This will be a problem later for Paul, but at this point we are only told that James is some sort of a leader in the Jerusalem church.
Some scholars have seen this passage as an indication that there is a shift in leadership in the Jerusalem community from Peter to James. This is possible (and I would even go so far as to say probable!), but it remains only a hint in this chapter. James, it seems to me, is a very significant leader in the Jerusalem church, although this fact is sometimes overlooked. Since Peter is the leader of the twelve most people look to him as a defacto leader of the church. I think this is a mistake, although Peter does continue to have some influence in Jerusalem (Acts 15).
It is significant that there is no effort to replace James the son of Zebedee after he is killed. On the one hand, it is 13 years after the resurrection, so the pool of individuals who could be witnesses from John the Baptist through the resurrection is likely very small – even James the Brother of Jesus does not qualify as a witness under those requirements!