One thing the party I was in struggled with was motivation to go to hell. The adventure is set up in such a way that a party in the first part may want nothing to do with being in hell, and a party that wants to save the world may not have been caught up in the first part of the adventure. This caused one person to change characters because they didn't fit in with the current part of the adventure and a lot of in character discussion about if we would want to go to hell. Find out what your group wants and maybe modify the adventure at the start to fit their style.

Another thing to play up is that a certain devil trapped in an object can also talk (and see) many of the creatures the party encounters and may use it to their advantage, if necessary.

I would also drop hints about the ending / freeing from service to avoid what happened in my group where one character refused to hand over the object, and ruining any chances for redemption. To be fair, their character didn't know what needed to be done, even though it was lightly hinted at.