Well, in regards to Morrigan, there are four things that need to be mentioned:
1. No matter what good deed you commit before Morrigan, you can always shower her with jewelry to make up for it. Seriously, I found 2-3 times as many gifts for her as for any other character, and this is supported by the DA Wiki...
2. The irony is, the more she likes you, the less of a ***** she is to you. Essentially, you need to be a bastard to get her to like you, but then she mellows out instead of you two being a pair of arseholes.
3. Clever use of dialogue and detailed looting for gifts means you can bed Morrigan as soon as you leave the first town. Yep, you can score as soon as you're out of Lothering...
4. She's voiced by Claudia Black. If she tells me to kill orphans, I do it and ask for more!