When I run a game that includes demons and/or devils The question, at least to me, arises "What motivates these things?"
To me, it come down to being hungry. Devils eat souls and demons feed on pain. Devils will try to lie, cheat, steal, bargain for and otherwise attempt to obtain souls. Ultimately the point is to consume them. sure, let them roast, toast, stew, simmer and otherwise torture them until it's time, but souls are on the menu when it comes to devils.
Demons feed on pain, suffering, fear, hate, and other extreme emotions. Once they are done with someone they leave them dead, useless husks. Maybe. The occasional demon has been known to take a dessert of flesh afterwards.
The point is, in roleplaying Devils and demons, motivation is key. These things don't just do what they do "just because". They are fulfilling a very real need for themselves.
I think of devils and demons who don't get to "eat" for long periods of time because they are confined to hell or the abyss or wherever as very nasty, grumpy, hasty creatures with one thing on their mind.
Those that get out to eat on some kind of frequent or infrequent intervals will be more practical and maybe even easier to bargain with.
Those that eat perhaps too often, are greedy and gluttonous like to play with their food. They will be most likely to make deals and interact because their food is also their entertainment.
Keeping all of this in mind is the basis for how I have demons and devils in games with humans and others.