Laws of Kedusha - Part 2
The congregation should not recite “Nakdishach…” with the chazzan. Instead, they should remain silent and pay attention to the chazzan, and when he reaches “Kedusha” they should answer aloud. (Shulchan Aruch Orach Chaim 125:1)
The Mishneh Berurah explains that the reason for this is that the Rabbis instituted that the chazzan should represent the congregation when he recites the introduction to Kedusha. Therefore, if they recite the introduction as well, how can we call the chazzan their shaliach (representative)? However, today the custom is not like the Shulchan Aruch. Rather, the congregation recites the entire Kedusha, the same as the chazzan. This custom is based on the Taz, though its widespread acceptance can be attributed to the fact that it is also the opinion of the Arizal. In fact, according to the Arizal the congregation should say each word with the chazzan. (See Mishneh Berurah, Kaf Hachaim, Piskei Teshuvot)
When Kedusha is said, one should join his feet together as though they are one foot, as is written about the angels (in Yechezkel 1:7): “And their feet (plural) are a straight foot (singular).” It is written in Sha’ar HaKavanot that one should be very careful about keeping his feet together during Kedusha. It is also written there that one should take three steps forward to recite Kedusha in the same place where he prayed Shemoneh Esrei.
When saying the three verses of Kadosh… Baruch… and Yimloch… one should raise his body and heels. It is the custom to raise one’s eyes upward (Rema), and preferably they should be closed (Arizal, Kitzur Shulchan Aruch). The Maharil would not speak from the beginning of Kedusha until the end of the blessing of “Ha’El HaKadosh”, and therefore one should also not speak at all during the Kedusha. One who already said Kedusha should nonetheless say it again if he is in a place where others are saying it (Rema). One who already said Kedusha and is in a place where he hears both Kaddish and Kedusha being said at the same time, answering “Amen Yehei Shemei Rabbah” for Kaddish takes precedence over Kedusha; but if one already started to say Kedusha he does not interrupt to answer “Amen Yehei Shemei Rabbah” (see Kaf HaChaim 125:17).