The UK site Scam Detectives has a fascinating interview with a convicted Nigerian email scammer:
Scam-Detective: How did you find victims for your scams?
John: First you need to understand how the gangs work. At the bottom are the “foot soldiers”, kids who spend all of their time online to find email addresses and send out the first emails to get people interested. When they receive a reply, the victim is passed up the chain, to someone who has better English to get copies of ID from them like copies of their passport and driving licenses and build up trust. Then when they are ready to ask for money, they are passed further up again to someone who will pretend to be a barrister or shipping agent who will tell the victim that they need to pay charges or even a bribe to get the big cash amount out of the country. When they pay up, the gang master will collect the money from the Western Union office, using fake ID that they have taken from other scam victims.
The full interview is worth checking out.