God is not limited in the way He works, He often does His work in a variety of ways. Here, in Acts 12, God sends an angel to release Peter from prison. Later, God will release Paul and Silas from the Philippian jail, but instead of sending an angel to open the doors, He will send an earthquake to shake the walls down. Our Lord is a creative God. Never try to put Jesus in a box. Be open to what His Spirit wants to do. If He wants to work in a new way, praise Him! As children of God we should be open to the moving of His Holy Spirit.