The person who is listening must be careful to answer Amen after each and every berachah of the Amidah repetition, as the Chachamim teach (Berachot 53b): “The one who answers Amen is greater than the one who recites the blessing.” One must respond Amen with the utmost solemnity, and while saying it, he must have kavanah that the content of the berachah is true. For instance, if one hears the berachah, “SheHakol Nihiyah Bidvaro,” he must have in mind that everything truly comes into existence through God’s word. When there is a request in the berachah as well, such as in Birkat Chonen HaDa’at, he should have in mind its two meanings: 1) it is true that knowledge comes from Hashem; 2) the request that Hashem endow us with knowledge (Shulchan Aruch 124:6; Mishnah Berurah 25).
One may not recite Amen too hastily, by saying it before the chazan finishes the berachah; nor shall one abbreviate the Amen by swallowing its letters, or pronounce it softly and in a weak voice. Also, one must not distance the response of Amen from the conclusion of the berachah, for then it is called an “Amen yetomah” (an orphaned Amen) (see Berachot 47a; Shulchan Aruch 124:8).
The way a person responds Amen represents how he relates to belief in Hashem. When a person’s faith is damaged, the life that Hashem bestows upon him is also impaired. This is what Ben Azai means when he says, “Whoever answers an ‘Amen yetomah’ (an orphaned Amen) – his children will be orphans; an ‘Amen chatufah’ (one that is too hasty) – his days [upon this earth] will be snatched from him; an ‘Amen ketufah’ (an abbreviated Amen) – his days will be shortened. However, anyone who extends the recital of Amen – his days and years are lengthened” (Berachot 47a).
The recital of Amen should be made in a pleasant tone and one’s voice should not be raised above that of the chazan who is reciting the blessing (Shulchan Aruch 124:12). One must not shorten the Amen, rather slightly elongate it for the amount of time it takes to say “Kel Melech Ne’eman,” although it should not be overly extended. If there are people who do prolong their response of Amen, the chazan need not wait for them; instead, after most of the congregation has finished saying Amen, he may continue on to the next berachah. However, regarding berachot with which the chazan fulfills the obligations of others, such as before the blowing of the shofar, the one reciting the blessing must wait until everyone finishes saying Amen so that those who extend their Amen can also hear the next berachah (Shulchan Aruch 124:8-9; Mishnah Berurah 38).
Prominent Rishonim were accustomed to saying “Baruch Hu u’varuch Shemo” after the recital of Hashem’s Name in the berachah, and their minhag became widespread throughout the nation of Israel. This applies to berachot concerning which one does not fulfill his obligation through someone else, such as the Amidah repetition for one who prays silently. However, concerning berachot whose obligation can be fulfilled through another person, such as the berachot of Kiddush and blowing shofar, the common custom is not to recite “Baruch Hu u’varuch Shemo,” so as not to interrupt the berachah by reciting words not established by the Chachamim. Nevertheless, b’dieved, if one responded “Baruch Hu u’varuch Shemo” to a berachah that he was obligated to recite, he still fulfilled his obligation, since his response did not distract his mind from the berachah (Mishnah Berurah 124: 21; Kaf HaChaim 26).
It is proper to enhance the mitzvah of listening to the Chazarat HaShatz by standing, just as the law requires for the silent Amidah. This is because even the listeners who do not fulfill their obligation by hearing the repetition participate in its recital to a certain extent and are considered as someone who recited an additional prayer. However, this is not an obligation, and one who wishes to sit is permitted (Mishnah Berurah 124:20; Kaf HaChaim 24).