I usually planned a main topic for each session in advance (during the week prior but prepared at least the night before) and jumped into it straight away. Then let the details and conversation naturally unfold and spread out on the go. I never really had problems not being able to speak about something when I decided that I wanted to discuss it, it was more avoiding certain things altogether (not even having the intention). Sometimes the T and I had different interests about a certain topic. No need to warm up for me at all, just to plan the main topic(s).