I was walking around WHOLE map: New Ark, Desert base, milk farm... Just walking with hope to trigger character Cassie to go outside and Gin to go to lake. I said about sleeping with hope that it matter AT LEAST something as well.
I have no idea how EXACTLY scripts working, but I HOPE I'm triggering event's correctly
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I have a much more efficient method. First, train Gin. If you're already in the 2 roleplay options and you're aiming for the third scene, make him wear a maid costume. Afterwards, leave the town and sleep.
If you get back and he's gone, don't go into the lake yet. Check the safehouse first if Cassie is there. If she's there, leave the town and enter again. Gin will still be in the lake so you just need to check the safehouse every time you enter. If Cassie is finally gone, you can check the lake and there's a huge chance they're both there.
You can also check the apple trees for Cassie first to make sure but I find that a bother and just prefer to save before checking the lake and reloading if she's not there.