Or HaChaim on Numbers 23:5/אור החיים על במדבר כ״ג:ה׳

1 וישם ה׳ דבו בפי בלעם, G'd put words in Bileam's mouth, etc. Although most commentators have already treated this verse exhaustively, they have also left some room for our comments. G'd wanted to use this opportunity to reveal part of the future and to mention the wonderful things that would happen to Israel at that time. He was particularly interested that this future be revealed to the Gentile nations by their own prophet. This is why He chose Bileam as His instrument to predict both Israel's eventual greatness and the other nations eventual downfall at the hands of Israel. When the Gentile nations would be able to note that one of their own had predicted all this it would impress them all the more. Due to the negative spiritual influences Bileam had surrounded himself with, the Holy Spirit which would enable him to foretell the future could not come to rest on him; not only this, but the words of G'd themselves are inherently sacred and not entrusted to a member of an impure nation. This is why G'd had to resort to a special stratagem so that words of holiness would not be spoken in impure surroundings. G'd constructed a barrier between the power of the speaker and the words he spoke, and the "mouth of the pig." This is what the Torah means when it writes: "G'd put a thing, דבר, inside Bileam's mouth." The דבר was the artificial barrier between G'd's holy words and Bileam's mouth. In this way Bileam's mouth was converted into a domain all by itself, divorced from Bileam the person. When the Torah continued וכה תדבר, the meaning is that with the help of this barrier in his mouth Bileam would be able to speak the words of G'd. The Zohar volume three page 210 writes that the word כה is an allusion to something sacred. Students of the Kabbalah will understand what I mean.

2In the event that you do not understand the nature of this separation within Bileam's mouth, consider Chagigah 15 where the Talmud explains how G'd had divided between different great bodies of water although they are adjoining one another without any visible barrier. The separations between these bodies of water are as thin as a hair. We are also taught in Shabbat 35 concerning the well which accompanied the Israelites thanks to the merit of Miriam that if someone wishes to see it he should climb Mount Carmel and look down into the sea where he would observe something that looks like a sieve. This is the well that used to accompany the Israelites in the desert.

3שוב אל בלק, "go back to Balak, etc." G'd forced Bileam to say the unpalatable things he was going to say in Balak's presence.