Heart of Darkness has this exploration/find the person situation, but you need to generate Beliefs for the characters that ties them into the situation and to each other. The start of the adventure does not seem to spend much energy explaining how the group are together or want to be involved, bar the basic 'here's the adventure lets do it'. BW needs a bigger buy in.
You're right - I made no effort to create any backround for the characters that might have led to them having a common interest in the opening scene of HoD. There are too many potnetial GMs out there (with too many diverse groups of players) to do that. Or am I missing the point about what you are missing?
But I still need some way to explain how the group is together and offered the job. Unless there was no group and each person has either been ordered to go, or learned about the job and got themselves to the meeting etc.
If I was starting a group from scratch - so they have no histroy together as Peter's group obviously has then I'd adopt something like the approach in the most recent incarnations of FATE. It uses a pre-game that helps generate links between characters, and also generates the 'Aspects' they want to see used by the GM. It might be that those relationhsips and aspects are driveres ot PC action that come close to the 'Beliefs' you're seeking.
I'd then seek to find ways to link the drivers the PCs define to the story being told in the adventure.
For instance - the pregame helps a player refine the character concept to include 'Conflicted Laranian' - brought up in a manor that changes hands between Rethem and Kanday during Ezar's War. His parents are subject to forced conversion to Laranianism and he grows up a zealot under the eye of a local priest drafted into the manor to ensure compliance with the new doctrine.
However, his parent continue to think of themselves as Rethemi, and the Laraniansim he learns under the birch rod of the priest is complemented with a sense of lost Rethemi-ness and even an unfocussed longing for his Rethemi heritage.
Now, to connect that to HoD: He might find himself travelling North to see whether 'the old country' is everything his parents described. They tell tales of the noble savage, Arlun, who shaped Rethem and the young man seeks knowledge by travelling north to meet some Kubora for himself. HoD becomes a journey of discovery among the Kubora where he is driven by curiosity about ther culture as much as the 'mission' as described. All he needs is an excuse to go upriver, but will he be fascinated or disgusted by what he discovers.
Consider a very different character, brought up in the slums of Golotha and absorbed into a life of crime. Perhaps he's sent, or perhaps he escapes, to Tormau to seek employment fomr the local Guild master. He's already working for Jamys when the strnager with a funny South Rethem/North Kanday accent arrives in town asking questions about the Kubora and their culture. He is drawn into the adventure as Jamys' eyes and ears - though that may not be something he shares with the other PCs.
What about a Knorran acolyte/priest? Perhaps easiest of all - she's in search of knowledge about something called 'Song of the Fallen God' which is referenced in a dusty tome of letters from a Corani commander in Peran to his wife at home. 'It seesm to me these savagaes will beleive anything at all ... they even think some lake or marsh in the wilderness was caused by a god falling out of the sky. Imagine it!'.
And finally ... my summary of hooks to save you reading HoD - though you'll need to before starting to play anyway, I'd suggest.
the blurring of myth and history;
desperation to get away from civilisation (for a while).
a missing person enquiry
Root of all Evil is premised on the PC group having completed HoD, so once they are in at the start it shouldn't be difficult to motivate them into the second episode. It wouldn;t be right to include too many spoilers, but the same drives would apply as above.