Here is how to get astarion approval so high he will proposition you before the tieflings party. No evil choices, just some chaos. The score you want is 40.
Not all of these are necessary btw, if you di all of this you'll get over 40. Avoid what sounds too evil and check that you're getting up to 40. Also remember that for the party romance you actually only need 20.
- When Lae'zel is in the cage, talk to her before releasing her and ask her to say "please"
- Attacking the thieves at the ruined temple
- When Shadowheart mention needing a cure (at camp) make sure to choose the option about being cautious
- When you meet with Zevlor, refuse to help saying you have your own problems. (you can help anyway, don't worry)
- When Lae'zel meets the tiefling and ask him to bow, go along with her at least once, then you can stop her and apologise to the poor man
- When you meet Wyll, you can talk with the guy training the children. Tell the children that they will all die
- After saving Arabella, tell the druid woman that you just wanted to see what would happen
- When you meet Auntie Ethel at the refugees' camp, let her fuss over you and then tell her about the tadpoles
- You know the kid with the magic ring trick? Pocket that ring
- When you meet the two siblings mourning their dying brother, Astarion will approve of you send them to face the owlbear (but I never choose this, because it makes me sad). But I think the big approval comes in the dialogue just next, where you can say that using the tadpole seems like a great idea
- At the blighted village, when the goblin spots you, you will get approval if you attack them back
- Say boooo to Volo
- Open the door where the sexy moans are happening
- At the entrance of the goblin camp, if you can, thow shit at the guard
- There's a goblin that will ask you to kneel and kiss his foot. Uno reverse it, and intimidate him
- Let the cleric at the goblin whip you
- After you free the owlbear, pet them at camp
- Get the necromancy of thay, and the amethyst that fits in it and give it to Astarion
- Recruit Karlach with him in the party
- Killing the monster Hunter will also shoot up your approval of TEN POINTS. I usually never kill him, but if you're close to the party and you really want to get to 40, this is also an option
- When you meet Raphael, agree with Astarion that you should not trust him
- If you decide to do auntie ethel too, make sure to convince her to free Maryna and get the deal at the same time
Now, I think the majority of approval will come with how you talk with him at camp.
- Go along with his dialogue about killing you if there's any tentacle showing. Just play along, don't shut him down
- Go along with the "whose blood would you drink" dialogue. Your answer doesn't really matter as long as you don't shut him down
- When he bites you, of course, offer to give him your blood
- During the mirror dialogue (sometimes it happens pre-sleeping together) you can insult him a bit (you're not aging gracefully), but at the second reply reassure him. The last reply is fine for all the lines and they seem to all get approval, as long as you don't just leave.
In general this is usually all I need to get to 40, which is pretty high. In my last playthrough I managed to also get to 40 without doing many of these.
But as I said, you only need 20. Just remember that he likes being petty and asserting dominance over others, he loves the tadpole, he is cautious, he's a little silly and chaotic, he's selfish, he likes joking and talking so don't shut him down.
Just remember that he's much more explicit about sex being an exchange if you get the proposition before the party, and you end up missing the scene of the party. Up to you! If you see that he's at 40 he will proposition the moment you talk to him, so if you accidentally got his approval too high and want to see both scenes you can technically kill the goblin leaders and free halsin, talk to him, then when you sleep you get directly to the party where you will have the party scene too.
