For what it's worth, there seem to be three triggers which get (a version of) that scene:


1. Talking to the drow in Moonrise
2. Completing Astarion's Act II personal quest
3. Having very high approval with him by some point in Act II (this is sort of a failsafe since it's very possible for the first two not to happen)

Unfortunately these can be messed up by bugs or by not long resting enough.