As soon as Act II begins, walk towards the door. You will have a short cut scene. Skip it if you want to, or watch it. Does not make a difference. After it ends, you will be outside the door, and Didi will be beside some pictures that have a glowing light in front of them, indicating that you can interact with them. Do so. Then, do not skip the cut scene. Watch it all the way to the end, and you will get this achievement.
Short video guide showing the cut scene that you need to have watched and the picture you need to have interacted with to get the achievement.
When this video ends, is when your achievement should pop.