Jonathan McLatchie talks us through at least a half-dozen appearances of the angel of the Lord in the Scriptures. Certain common threads can be seen. He begins his long post in this way:
Throughout the Old Testament, we routinely encounter the mysterious character who goes by the title “The angel of the Lord.” By looking at the numerous appearances of this individual, we can piece together clues as to His identity.
This post also shows up on Christian Apologetics UK.