One does not interrupt his Amida to answer either Kadish or Kedusha. Rather, one should remain silent and have Kavvanah to what the Hazzan is saying. This will be as if he himself is answering the Kadish or Kedusha, by virtue of the fact that he is listening intently.
However, if one is up to the section אלהי נצור at the end of the Amida, after the first pasuk of יהיו לרצון is recited, one can answer Kedusha and Kadish and Barekhu.
For Kedusha--answer only the pesukim beginning קדוש and ברוך;
Kadish--answer only the first five Amenim, and יהא שמיה רבא until עלמיא יתברך;
Barekhu--the full pasuk ברוך ה' המברך לעולם ועד
Modim deRabbanan--only the first three words מודים אנחנו לך
However, if one is up to the section אלהי נצור at the end of the Amida, after the first pasuk of יהיו לרצון is recited, one can answer Kedusha and Kadish and Barekhu.
For Kedusha--answer only the pesukim beginning קדוש and ברוך;
Kadish--answer only the first five Amenim, and יהא שמיה רבא until עלמיא יתברך;
Barekhu--the full pasuk ברוך ה' המברך לעולם ועד
Modim deRabbanan--only the first three words מודים אנחנו לך