In Greek the adverb actually is literally “but.”
He did not say this of his own accord (so also Revised Standard Version, New English Bible; Moffatt “… simply of his own accord”) appears in a variety of ways in different translations: “Now he was not self-moved in saying this” (Goodspeed); “He did not speak in his own person” (Jerusalem Bible); “He did not make this remark on his own initiative” (Phillips). The form of the word here is similar to that of the words of 14.10; The words that I have spoken to you … do not come from me. John is saying in these two passages that God is able to speak through a person, whether that person is conscious of it (as in 14.10), or not (as here). He did not say this of his own accord may be expressed in some languages “He didn’t himself think up this” or “He himself didn’t think of this which he said.”
Jerusalem Bible omits the phrase that year from this verse, though the manuscript support for the omission is very weak. Its omission from one Greek manuscript is perhaps due to the observation that it was redundant after verse 49.
The Jews traditionally ascribed a degree of prophetic ability to the High Priest, and that belief is reflected in this verse. Despite Caiaphas’ unwillingness to respond to God’s words in Jesus Christ, God was able to speak through him, without Caiaphas’ realizing it. As he was High Priest that year should be treated in most languages as causative, for example, “because he was the High Priest that year.”
The verb “to prophesy” in a biblical context normally has the primary meaning of “to speak forth God’s message,” but in the present context the meaning is obviously “to predict.” In this context he was prophesying may be rendered “he was telling ahead of time what was going to happen; he was saying that Jesus was going to die for the Jewish people.”
As indicated in the preceding verse, the phrase for the Jewish people is literally “for the nation,” but the meaning is the same as the word translated people and nation in verse 50.
Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .