I thought I'd list all of the general topics that you can ask anybody about, and how to unlock them. Maybe it would be of interest to those who want to hear about everything from everyone, or to those looking for that last lousy point. (All of the actions done to unlock these topics give you points.)

I'm sure that there are other ways to unlock some of these topics, but I'll stick with what I know from my playthrough:

Voodoo: From the beginning
Voodoo Murders: From the beginning
New Orleans: From the beginning
Cabrit sans Cor': Ask Willy about Voodoo Murders OR show Willy the photo of the murder victim.
St John's Eve: Read the lagniappe sign in the Dixieland Drug Store.
Snakes: Pick up the snake scale at Lake Ponchartrain.
Marie Laveau: Ask Dr. John about Historical Voodoo until he has nothing more to say.
Animal Masks: Ask Magentia about St. John's Eve.
Schattenjager: Read the letter to Heinz Ritter.
Veve: Attend the lecture at Tulane.
Black Voodoo: Show Professor Hartridge the photo of the murder victim.
Voodoo Hounfour: Ask Mme. Casaunoux about the Real Voodoo Queens.
Rada Drums: Ask Crash about the Drummer.
Damballah: Read Professor Hartridge's notes about the veve.
Ogoun Badagris: Read Professor Hartridge's notes about the veve.