The Shulchan Aruch (O.C. 128) writes, based on the concluding passage in Masechet Rosh Hashanah, that the Birkat Kohanim blessing recited by the Kohanim in the synagogue is ineffective with respect to people standing behind the Kohanim. In introducing the Mitzva of Birkat Kohanim, the Torah (Bamidbar 6:23) writes, "Amor Lahem" – "Say to them." This implies that the Beracha must be recited in the manner in which people speak to one another. Since one generally faces the person to whom he speaks, the Kohanim must likewise face the congregation when they declare this blessing. Hence, those standing behind the Kohanim must move in front of the Kohanim to ensure that they are included in the blessing. The Shulchan Aruch (128: 23-24) emphasizes that those standing in front of the Kohanim are included in the blessing even if an "iron barrier" separates between them and the Kohanim. So long as they are positioned in front of the Kohanim, they are included in the blessing regardless of any object that stands in between them.
Furthermore, the Shulchan Aruch rules that those directly to the side of the Kohanim are considered to be in front of the Kohanim for the purposes of this Halacha. Chacham David Yosef, in his work Halacha Berura (vol. 6, p. 432), adds that those standing directly to the sides of the Kohanim should turn their faces towards the Kohanim during the blessing, so that they will be facing them as required. However, he noted, people situated to the side and in front of the Kohanim, on a diagonal, need not turn their faces towards the Kohanim.
The Shulchan Aruch (ibid) adds that those who cannot come in front of the Kohanim due to extenuating circumstances, such as ill or otherwise frail individuals who cannot reposition themselves, are nevertheless included in the Beracha. Since it is due to circumstances beyond their control that they cannot come in front of the Kohanim, the Beracha applies to them even though they stand behind the Kohanim. The same applies to those who cannot come to the synagogue due to extenuating circumstances, such as those who are preoccupied with their work; they are included in the Kohanim's blessing even though they cannot attend services. The Kohanim must therefore have in mind when reciting the Beracha that it applies not only to those who stand in front of them, but also to all those who sincerely wish to come to the synagogue but are unable to do so.
Clearly, however, those who do not come to the synagogue due to laziness or for their own convenience are not included in the Beracha.
Summary: During Birkat Kohanim, everybody must ensure to stand either to the side or in front of the Kohanim, and not behind them. Those standing directly to the sides of the Kohanim should turn their faces towards the Kohanim as they recite the Beracha. The Kohanim should have in mind when reciting the Beracha that it applies even to those people who sincerely wish to come to the synagogue but are unable to do so.
