When the Kohanim recite the blessing, the people being blessed must position themselves in front of them, as it is written (Numbers 6:23), “This is how you must bless the Israelites. Say to them.” Chazal interpret this to mean that Birkat Kohanim must be performed in the same way that people talk to their friends – by standing face to face and speaking aloud – so that all those receiving the blessing can hear them.
Although the Kohanim reciting the blessing must stand, in principle, those being blessed may sit. Nonetheless, today, the custom is that everyone stands for Birkat Kohanim. Still, an ill or weak person who has difficulty standing is permitted to sit for the blessing (Mishnah Berurah 128:51; Tzitz Eliezer 14:18).
Anyone standing behind the Kohanim is not included in the berachah, although one who is standing directly beside them can be included as long as he turns his face towards the Kohanim. People sitting in the first rows of the synagogue must measure their place in relation to the Kohanim. If they are in front of them or even directly to the side of them, they are permitted to remain in their place and turn their face towards the Kohanim. However, if their place is slightly behind the Kohanim, they must move to a different position for the recital of the blessing (Shulchan Aruch 128:24).
Anyone standing in the synagogue in front of the Kohanim is included in the berachah. Even if there are tall people standing before him, separating him from the Kohanim, or if there is a pillar between him and the Kohanim, since he is on the side that is across from their faces, he is included in the berachah. However, someone who stands in front of the Kohanim and turns his back to them is not included.
Those who do not come to synagogue due to circumstances beyond their control are still included in the berachah. For instance, someone who has to leave for work, or women and children who do not come to synagogue, are all included in the berachah, which is intended to include all of Israel. Only those who are able to go and receive the blessing, but neglect to do so, are not included in the berachah (Bei’ur Halachah 128:24 s.v. “Im”).
. One who is in the middle of reciting Shemoneh Esrei and is standing behind the Kohanim may not interrupt his prayer to walk and stand in front of them. This is considered as a circumstance beyond his control, and therefore he is blessed where he stands. The chazan acts similarly; even when the Kohanim are behind him he does not go stand in front of them. Even though walking is not considered a complete interruption during the Amidah, still, it is prohibited unless necessary. Therefore the chazan is treated as one who has circumstances beyond his control and is therefore blessed where he stands. (The Igrot Moshe Orach Chaim, part 5, 20:23 writes that one who is in the middle of the Amidah should walk in front of the Kohanim. Yet, in part 4, 21:2, at the end of his ruling, he writes the exact opposite.)
One who is in the middle of reciting the Amidah when the congregation reaches Birkat Kohanim should be silent and concentrate on the berachah. Regarding Kedushah and Amen yeheh Shemei rabbah, he is not obligated to stop if he does not want to. However, in this case, where some poskim maintain that the Israelites who are being blessed also fulfill a biblical mitzvah, he must stop and listen to Birkat Kohanim, although he may not respond Amen. If he is at the same part of the prayer as the chazan, he answers Amen to the three verses of blessing. Still, concerning the berachah recited before them, some say he may not respond Amen. See Mishnah Berurah 128:79, where he rules this way regarding the chazan.