In terms of NPC interaction, I've taken the Mass Effect approach. Between core story missions, you return to your home base of Fort Velen. This helps drive some key events and discussions vital to the plot, and helps with the pacing and overall flow of the game. The player does get a choice of tasks they wish to pursue, so having them return to Fort Velen allows me as the designer to bring certain essential items to the player's attention should they not have discovered them on their own.
You'll become familiar with the surroundings of Fort Velen
This design means that most of the major interaction between party members will occur in Fort Velen. Outside of Fort Velen, they will generally only provide 1-line responses to any attempt to talk to them. I understand that many players dislike this model, but given the number of potential choices and scenarios they player can be in, I simply can't justify the overhead of the dialogue (particularly VO work) that the alternative would require.
However, in addition to party members, there will also be a number of key NPCs who are part of the campaign against the darkspawn who are based in Fort Velen. Your interactions with them will occur here as well - though in all these situations, you will be given choice. These NPCs may come to you with issues or opinions and ask for your support, and your decisions here will affect how they react to you, and may result in them providing or withdrawing their help to you. You may even choose to ignore them, which could bring its own consequences.
Allies or adversaries? That's up to you...
One thing I am not doing is making the ramifications of those choices blatantly obvious. One of the design decisions I disliked from The Witcher was the way the game informed the player very clearly about a consequence related to an earlier decision. While some players appreciated being told exactly what result their decisions had, I found it an immersion breaking technique. Every choice the player makes should lead to a story that feels unique and accurate - not one of a series of alternate realities that are made blatantly clear. While the illusion can be broken somewhat by multiple playthroughs of a game, I believe that each should feel like it was the "true" story. To do otherwise is to partially rob the player of the power of telling their own story.
So there's an insight into the design aspects of interactions with major NPCs in The Shattered War. If you have any specific questions or things you'd like to know, please comment and I'll provide a response!