Hilchos Kaddish
 
1052. One who answers "Amein Yehei Shemei Rabbah etc." with all his strength (i.e. Kavannah - concentration); has his bad decree torn (in heaven), and the gates of Gan Eden are opened up for him, and even if he has a smidgen of sin of Avodah Zorah he is forgiven. Shulchan Aruch w/Mishnah Berurah 56:1 MB1, Mes. Shabbos 119b
 
1053. One should carefully listen to the words of the Kaddish from the Chazzan in order to know to what one is answering Amein, Amein Yehei Shemei Rabbah, or B'rich Hu. Shulchan Aruch w/Mishnah Berurah 56:1 MB1
 
1054. One is not permitted to speak during Kaddish and one must be very, very careful not to speak during Kaddish or Kedusha. Shulchan Aruch w/Mishnah Berurah 56:1 MB1
 
1055. The Mishnah Berurah brings a story of a Chosid (i.e. a person scrupulous in the performance of mitzvos) who saw another fellow Chosid after he died and his face was discolored green. He asked him "Why is your face green"? The Chosid responded "Because I used to talk while the tzibbur was davening Vayechulu (on Friday night), and during the beracha of Magen Avos (on Friday night), and during Kaddish". Shulchan Aruch w/Mishnah Berurah 56:1 MB1
 
1056. The Mishnah Berurah then brings a story from a Midrash; A certain Chacham who had died appeared to his student in a dream and the student saw that his Rebbe had a stain mark on his forehead. The student asked "Why did this happen to you"? The Rebbe responded "Because I was not careful to refrain from talking while the chazzan was saying Kaddish". Shulchan Aruch w/Mishnah Berurah 56:1 MB1
 
1057. One is not permitted to even think in Torah during Kaddish. Shulchan Aruch w/Mishnah Berurah 56:1 MB1
 
1058. Now that we are in Golus (exile) the world only remains in existence in the merit of the praise of Hashem pronounced by Klal Yisroel in the Kaddish. Piskei Tshuvos 56:1, Mes. Sotah 49a
 
1059. One should respond to Kaddish in a loud voice which arouses concentration, and via this loud voice one has the power to break bad decrees in heaven. Shulchan Aruch w/Mishnah Berurah 56:1 MB5
 
1060. However, one should not scream so loudly to cause others to make fun and chuckle derisively, as this would cause those others to thereby sin. Shulchan Aruch w/Mishnah Berurah 56:1 MB5
 
1061. Answering "Amein Yehi Shemei Rabbah" is greater than saying Kedusha or Modim, thus one should run to be able to listen to and answer a Kaddish. Shulchan Aruch w/Mishnah Berurah 56:1 MB6
 
1062. Therefore, if one is in a large shul with several minyanim praying simultaneously, and one hears Kaddish and Kedusha, one should listen to and respond to the Kaddish, rather than the Kedusha. Shulchan Aruch w/Mishnah Berurah 56:1 MB6
 
1063. L'chatchila one should make an effort to stand during Kaddish, at least until after saying "Amein Yehei Shemei Rabbah", and the Amein that follows it. Shulchan Aruch w/Mishnah Berurah 56:1 MB7,8
 
1064. However, according to Rav Shloma Zalmen Auerbach, (and many other Poskim) one should L'Chatchila make an effort to stand for the entire Kaddish. Halichos Shlomo 86, Sdei Chemed 2:29, Piskei Tshuvos 56:4
 
1065. If one arrives in shul in middle of Kaddish or Kedusha one should answer with the Tzibbur even if one did not hear the words of the Chazzan. Shulchan Aruch w/Mishnah Berurah 56:1 MB9
 
1066. One should answer all Amein's with the Tzibbur even if one is hearing Kaddish with a different Nusach than one's own Nusach. (e.g. Nusach Sefard that says Veyatzmach Pikornei). Piskei Tshuvos 56:5