Hopelessly Romantic Achievement
Across all playthroughs, experienced all possible romances
Complete the romance stories with Morrigan, Leliana, Zevran, and Alistair. For Morrigan, your character needs to be male. For Alistair, your character needs to be female. For Leliana and Zevran, your character can be either male or female. When you reach a certain level (track your progress with the meter below their Character Record), you will slowly approach the end of the romance. You need to get the meter as high as possible (Leliana for example is close to maxed out, Zevran doesn't take nearly as much). You'll do a quest for them, then share a kiss, and then...
For an easy way to gain points with them, go into your inventory, and scroll over to the "Other" list. Anything that says "Gift" in purple can be given to any potential romantic interest. Just use to scroll over to the character you're interested in, highlight which item you're going to give her, and press to give it to him. The best items vary from character to character. See "First Knight", "Witch Gone Wild", "Easy Lover", and "Wine, Woman, and Song" for more details.
You'll probably need two playthroughs for this, but it may be possible to do this in one. If you have a high enough affection with each of the four, have been romantic with at least one of them, none of them are jealous, and have a high enough coercion talent and cunning ability, then you may be able to pull off a foursome with Isabella in The Pearl in Denerim.