If a person hears a Beracha recited during the Pesukeh De’zimra section of the morning prayer service, he should interrupt his recitation in order to answer "Amen." It often happens that as a person prays Pesukeh De’zimra, he hears somebody near him recite the Beracha over the Tallit or Tefillin. In such a case, he should answer "Amen," though he should not respond "Baruch Hu U’baruch Shemo" in the middle of the Pesukeh De’zimra.
During Pesukeh De’zimra one also answers to Barechu. If he hears Nakdishach during Pesukeh De’zimra, then he interrupts to answer "Kadosh Kadosh Kadosh…" and "Baruch Kebod Hashem." One does not interrupt Pesukeh De’zimra to recite the introductory passage of "Nakdishach" or to recite "Yimloch Hashem Le’olam." If one hears Kaddish during Pesukeh De’zimra, then he answers the first five "Amen" responses, meaning, the "Amen" responses until and including "Amen Yeheh Shemeh Rabba." When answering "Yeheh Shemeh Rabba" to Kaddish during Pesukeh De’zimra, he recites the entire response, through "Da’amiran Be’alma," as usual. Furthermore, if one hears Modim De’rabbanan during Pesukeh De’zimra, then he should slightly bow and recite the three words of "Modim Anahnu Lach."
The Pesukeh De’zimra section begins with the introductory blessing of Baruch She’amar. If a person completes Baruch She’amar before the Hazan, he should immediately proceed with "Mizmor Le’toda," and then answer "Amen" to the Hazan’s Beracha when the Hazan completes Baruch She’amar. If he completes Baruch She’amar together with the Hazan, then he should not answer "Amen," as this would give the appearance of answering "Amen" to his own Beracha.
The Halacha for interrupting during Baruch She’amar itself depends on which part of Baruch She’amar one is reciting. If one has already begun the Beracha ("Baruch Ata Hashem Elokenu Melech Ha’olam Ha’Kel Ha’ab…"), then he may interrupt for any response for which one may interrupt during Pesukeh De’zimra. Even though he is in the middle of a Beracha, he may interrupt for the responses mentioned above. If, however, he is still in the earlier part of Baruch She’amar, before the Beracha, then he interrupts for any response, including for the final "Amen" responses of Kaddish (such as for Titkabal, Yeheh Shelama, and Al Yisrael Ve’al Rabbanan). The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) writes that in such a case, one should return to the beginning of Baruch She’amar after interrupting. The eighty-seven words of Baruch She’amar should preferably be recited without interruption, so if one had to interrupt before the Beracha, he should return to the beginning so he can recite the entire text without interruption.
Summary: During Pesukeh De’zimra, one may interrupt to answer "Amen" to a Beracha, to join in the first five responses to Kaddish, for Barechu and for the "Kadosh" and "Baruch Shem" responses of Nakdishach. If one hears the Hazan recite Modim as he recites Pesukeh De’zimra, he should bow and recite the words "Modim Anahnu Lach." These Halachot apply as well if someone is reciting the second section of Baruch She’amar, from the point of "Baruch Ata Hashem." Before the point, one may interrupt for any response, but in such a case he should then go back to the beginning of Baruch She’amar.
