Halacha forbids walking within four Amot in front of somebody praying the Amida. Likewise, if a person prays the Amida in front of somebody else praying the Amida, and the person praying in front finishes the Amida first, he may not take his three steps back if he will then be within four Amot of the person praying behind him.
Hacham Bension Abba Shaul (Israel, 1923-1998), interestingly enough, makes an exception in a case where people are praying in two different domains. This happens in a situation where a person arrives late to the service, and recites the Amida just inside the doorway to the sanctuary. Meanwhile, as he prays his Amida, somebody else walks into the building, and begins reciting the Amida behind him, in the hallway. In such a case, Hacham Bension ruled, the person praying inside the sanctuary would be allowed to take his three steps back when he completes the Amida, even if he will be within four Amot in front of the person praying the Amida in the hallway. Hacham Bension claimed that since the two people are in different rooms one is in the sanctuary, and the other is in the hallway the prohibition against passing in front of somebody praying does not apply.
Hacham Ovadia Yosef, however, disagreed with this ruling. He noted that the reason why Halacha forbids walking in front of somebody praying the Amida is that this may likely disrupt his concentration on his prayer. Accordingly, Hacham Ovadia argued, it makes no difference whether the two people are in the same room or in two separate rooms; either way, one will be disturbing his fellows concentration by walking in front of him. Therefore, one may not take his three steps back in front of somebody praying the Amida, even if he is in a different room.
May a Kohen walk in front of somebody praying the Amida during Shaharit so he can wash his hands in preparation for Birkat Kohanim?
The Halacha in this case depends on whether or not the Kohen had ensured to keep his hands clean since he washed them before Shaharit. The Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) ruled that a Kohen does not have to wash his hands before Birkat Kohanim if he had made a point of keeping them clean since he washed Netilat Yadayim earlier in the morning. Although Halacha does not follow this opinion, and requires Kohanim to always wash their hands before Birkat Kohanim, a Kohen may rely on this ruling if he would have to walk in front of somebody praying the Amida to wash his hands. Therefore, if the Kohen had ensured to keep his hands clean, he should recite Birkat Kohanim without washing his hands rather than walk in front of somebody reciting the Amida to wash his hands. If, however, he had touched his shoes or, in general, was not careful to keep his hands clean, then he must wash his hands, even if this requires passing in front of somebody reciting the Amida. The Torah obligation of Birkat Kohanim overrides the prohibition against walking in front of somebody praying the Amida, and therefore, if one must wash his hands for Birkat Kohanim, he should do so even if this entails walking in front of people praying the Amida.
Another situation where we make an exception to this Halacha is a case where a person is needed to complete the Minyan, and somebody is praying the Amida by the doorway. One is allowed to pass in front of the individual praying the Amida if this is necessary for him to enter the synagogue to complete the Minyan.
Summary: One may not walk within four Amot in front of somebody praying the Amida, or take three steps back after the Amida prayer if he is within four Amot in front of somebody else praying the Amida. A Kohen who must pass in front of somebody praying the Amida to wash his hands for Birkat Kohanim should do so, unless he has ensured to keep his hands clean, in which case he should recite Birkat Kohanim without washing his hands. If a person is needed to complete the Minyan but somebody else is reciting the Amida by the doorway, the person may enter the room to complete the Minyan, even though he will be passing in front of somebody reciting the Amida.
