There are a number of instances in the OT where the angel and God (YHWH) are separate yet overlapped. There is the angel that bears God's name in Exodus 23:21
There is YHWH speaking to Moses in Exodus 3; then Stephen in Acts 7:30 says that an angel spoke to Moses.
Jacob wrestled with an angel, and wrestled with God also.
Manoah and his wife in Judges 13 saw an angel, who they called God.
In Genesis 16/21, the angel sees Hagar and she calls him "The God who sees me".
These angels are THE angel of God. Singular. The one who bears God's name. This is not Gabriel or Michael. This is the Angel of the Lord.
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Agency: this Angel is the Holy Spirit, which is God "economically" because this Angel is one with God. So when you see this Angel you are seeing God. When this Angel talks you are talking to God. So this Angel is God, economically, but not essentially.
The Roman Centurion in Luke 7 said that he was "a man under authority", as was Jesus under God. And the Holy Spirit is under Jesus. See John 15 and 16 for discussion of this -- in these 2 chapters, Jesus presents us the Father, and the Holy Spirit presents us Jesus. In the Gospels, when we see Jesus we see the Father, and in John 15/16, when we see the promised Holy Spirit we see Jesus. Just like when the Roman Centurion spoke, because he was one with Caesar, you heard Caesar through the Centurion. This is agency. This is extension of power, intent, kingdom, domain, through the agency of an intermediary. The key with Jesus and the Holy Spirit is that they are "next in line" from God, so when you get them you get God. Pure, unadulterated God. This is the idea of agency at its finest. Like if the Secretary of State goes to England, he can say, President Smith wants this. Because the Secretary is one with the President. When the Secretary speaks, you hear the President, first-hand.
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Further documentation: an angel told Philip to go down the south Road out of Jerusalem in Acts 8:26, then in verse 29 "the spirit" told Philip to run up to the chariot.
In Revelations 2 and 3 you have Jesus speaking to the angel of the church, then at the end of each message you have "hear what the Spirit is speaking to the churches". So the angel receiving in the beginning is the spirit speaking at the end.
See the pre-Trinity formula:
Whoever is ashamed of me and my words, the Son of Man will be ashamed of them when he comes in his glory and in the glory of the Father and of the holy angels. You have the glory of the Son, the glory of the Father, and the holy angels. No Holy Spirit mentioned here.
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Lastly, these ideas are not mine. They were in the discussions of the church fathers pre-Nicene. But the rise of the "orthodoxy" movement put an end to this. Also probably a lot of speculative literature got actively repressed. I am guessing the Book of Enoch among others.
Clement wrote a bit about this, as did Origen. The idea that the Spirit/Angel/Christ overlaps. Not always clear which is which. The only thing that is truly original is my idea that the Holy Spirit is the "doppelganger" of Jesus. It his his "other half". Like "spooky action from a distance".
http://en.wikipedia.org/wiki/Quantum_entanglement
Sorry, but I like physics. But there is a lot of scholarly documentation on the Church Fathers, Pre-Nicene, Pre-Trinitarian Creed, discussing the Spirit/Angel overlap. But once we got the Trinity Formula, most all of this got dropped. I simply revive it here because it makes more sense with the text than the Trinity formula. With the trinity formula, you have to ignore the text. I don't like that. Plus I like physics, and this formula allows me to include angels, principalities, powers etc in my view. Instead of the simple trinity formula, which essentially sheds all those essential details.