In the mini-Kedushah, when the lines of Kedushah are recited in blessing Yotzer Or before the morning Keriath Shema', and in Kedushah DeSidra (colloquially known as Uva Letzion and recited at various prayer services):
What is the purpose of the lead-in lines which are recited aloud by the Shaliah Tzibbur? Is it just to make sure everyone is, more or less, able to recite the lines in unison?
What is a congregant expected to do? If someone is up to this point with the congregation, should the person recite the responsive lines of Kedushah aloud, or can they be recited quietly; and if someone is not up to that point, but is able to recite those lines (eg., the person is behind or in the middle of a different part of Tefillah, but not something like Shemoneh 'Esreh or Shema') is the person expected to join the congregation, or can the person continue on without responding, as if praying alone?