Before the Kohanim recite the Birkat Kohanim, they first recite the introductory Beracha: "Baruch Ata Hashem…Asher Kideshanu Bi’kdushato Shel Aharon Ve’sivanu Le’barech Et Amo Yisrael Be’ahaba." The Shulhan Aruch (Orah Haim 128:11) writes that the Kohanim should all recite this Beracha together, in unison. One Kohen should not recite the Beracha on behalf of the others, and they should not recite it at different speeds; rather, they should all recite it together. It is customary for the Kohanim to begin the Beracha while their backs are still facing the congregation, and then turn around to face the congregation before reciting the words, "Le’barech Et Amo Yisrael…"
The Shulhan Aruch adds that since, as the conclusion of this Beracha ("Be’ahaba") indicates, Birkat Kohanim must be recited with feelings of love toward the congregation, a Kohen should not recite the blessing if he harbors feelings of animus toward the congregation. The Zohar comments if a Kohen dislikes the congregation, then reciting the Beracha may be detrimental for him. The same applies in the opposite case, if the congregation dislikes the Kohen. Therefore, a Kohen should recite Birkat Kohanim only if he is on generally good terms with the congregation; otherwise, he should leave the synagogue before the Hazan begins "Reseh."
Summary: The Kohanim must all recite the introductory Beracha to Birkat Kohanim in unison, and they turn around to face the congregation before reciting the words "Le’barech Et Emo Yisrael." If a Kohen dislikes the community, or the community dislikes a Kohen, he should not recite Birkat Kohanim.
