The earliest I could get this achievement to pop occurred after you emerged from the Deep Roads Expedition.
Upon returning to Kirkwall, I went to the Viscount, back home and read some letters, then to the Hanged Man. At the Hanged Man I spoke to Isabela, and she informs you that she has a lead on the relic she asked you to help her retrieve back in year one. One of the dialogue options says something along the lines of: "Do I get a kiss if I help you?"
As soon as you pick that dialogue option the achievement will pop up. I am a female so I can confirm it works for that gender, and I can only assume it would work for a male as well (seeing as Isabela is one of the few female party members)