Diablo has dialogue?!
Dialogue will be text based, voice would be too restrictive in terms of the quantity, choice and dynamic qualities of dialogue. That said we are still considering giving NPCs voices by having them speak greetings or short phrases. We potentially have access to a large number of skilled voice actors. We will get your feedback on this.
In terms of volume of dialogue we want to leave this largely up to the player. You can be very brief but if you push NPCs for more information the dialogue can extend significantly.
Ideally we want to give you a broad choice of things to say to anyone. This would be done by providing a large volume of possible preset questions and statements and a choice of attitudes to adopt. NPCs would react to what you're saying and how rather than just answer you. They might get annoyed even just by the volume or repetitiveness of what you say.
We've even seriously considered a natural language interface but this a little too ambitious perhaps!
Edit: Forgot to reply to hredthel. The basic idea is that actions count for more than words. You can influence people and change their attitude towards you but just saying something doesn't make it happen, doing it does. Same with telling / persuading someone else to do something, they might inted to do it but you or someone else could still prevent them. Anything could happen...