For AQA,
You pick one theme for your general conversation. You then get asked another theme in your general conversation, and the last theme comes up in your photo card. For example, if you pick Theme 3, your teacher will go across the row (on a sheet paper they have) that will say, for example, 2nd theme for General Conversation is Theme 1. This leaves Theme 2 for your photo card.
Role Play is just completely random, and sometimes encompasses 2 or all 3 themes, but they're simple, so it shouldn't matter too much.