Alfred Edersheim, The Life and Times of Jesus the Messiah, p.308 (Hendrickson:1993)
[Of the reading of the Scriptures in the synagogue] 
As the Hebrew was not generally understood, the Methurgeman, or Interpreter, stood by the side of the reader (comp. 1 Cor. 14:27,28), and translated into the Aramaean verse by verse, and in the section from the Prophets, or Haphtarah, after every three verses (Megill. 24a). But the Methurgeman was not allowed to read his translation, lest it might popularly be regarded as authoritative. This may help to us in some measure to understand the popular mode of Old Testament quotations in the New Testament. So long as the substance of a text was given correctly, the Methurgeman might paraphrase for better popular understanding. Again, it is but natural to suppose, that the Methurgeman would prepare himself for his work by such materials as he would find to hand, among which, of course, the translation of the Septuagint would hold a prominent place. This may in part account alike for the employment of the Septuagint, and for its Targumic modifications, in the New Testament quotations.