Karlach wakes you up one night to tell you that she wants "to ride you until you see stars", if I remember the wording correctly, while Shadowheart will invite you for a bottle of wine during the party if you haven't asked to get to know her better before. She will then disapprove if you don't turn the friendly night spend chatting romantic by kissing her. I think you can only have two active romances in Act 1 so some scenes maybe might not trigger - or at least Lae'zel tells you that you are missing out, when you have previously flirted with two other companions before she can make her move. ^^