1) All of the viduys must be recited while standing, and it is good to bend over during their recital like when saying “Modim”. One may not lean on anything in a way that if the supporting object were to be removed that he would fall. One must remain standing until after the words “Al Chataim she’anu chayavim aleihem arba misos beis din”. (Shulchan Aruch and Mishne Berurah 607,3)
2) One may not speak in the middle of the viduy, and not in the middle of “Al Cheit”. However, one may respond to Kaddish or Kedushah. This means that even in the middle of the main viduy (Ashamnu etc.) one must stop to respond to kaddish and kedushah. (ibid)