May a Ḥazzan decide on his own to do a Hechi Kedushah, or are there rules for when it is appropriate/mandated?
On shabbat for shacharit and mussaf, I am usually still davening shmoneh esre by the time the chazzan gets to kedusha. In this situation, can I hum to the tune of kedusha? This essentially boils down ...
When the chazzan starts kedusha in chazarat hashatz while you're davening, are you supposed to answer the chant, or continue in your tefilla, or just not say anything?