Ideas on introducing fiends in your campaign:
1. In your local town, intoduce the characters to a charismatic local (barkeep, blacksmith, thief for higher, tracker for hire). Present the character in a very likeable way. Turns out, of course, he is a fiend in disguise, but he's here for reasons that have nothing to do with the PC's. (yet)
2. In my campaign, there always voices in the night, attempting to find the weak and encourage them to commit terrible acts, thus damning their souls. Introduce the characters to a line of thought (a book, philosopher, whatever) that suggests that these are not really evil acts, it's just a sort of Darwinian effect, were only the weak succumb and the strong/righteous exist. Therefore, you can converse with Fiends only if you have resolve.
3. Another idea is that fiends, especiallyhigher level ones, do not wander around and eat everyones soul they see. They are here for a higher (evil) purpose, but are willing to make friends along the way for maybe later. Similar to idea number one, just less deception.
4. Introduce the PC's to a possessed character (see BoVD) and they get asked for help to kick the beast out. Turns out the beast is ust doing it because it's forced to by some bigger baddies, needs some item/task doen before it can release the victim and crawl back to Hell. PC's hopefully agree to help, they've intereacted with fiends and did something good.
BTW, all of the ideas scenarios in my mind would end up with the PC's eventually figuring about that Demons and Devils really are the living breathe of evil, and they were idiots for thinking otherwise when the devils were being so charming earlier. Have a PC possessed later, or have a family member kidnapped for a sacrifice to some dark demon prince. I'm just trying to suggest that just because they're evil, does not mean they are stupid. Hope this helps.