Semiazas,
Hopefully my reply can be of some help to you. If not, sorry. I tried.
I'm going to give you a rundown of how my mind works when it comes to writing story. I'm going to take your "serial killer" idea and use it for this example.
So you basically have a story about a serial killer. Somebody comes in an interrogates the serial killer. By conversation the serial killer leaves some kind of imprint on the person and I'm guessing the interrogator now goes out there and kills?
Alright, so you have your idea. Now let's break it down. In the world that we are telling the story, how on Earth is the serial killer going to be able to leave some impression on the interrogator to where he is in a trance and now goes out an kills???
Well, we have to establish this world a little bit. In my opinion we can do this in an indirect kind of way.
Let's clutter the serial killer's jail cell with some spiritual books. Some books on voodoo and dark magic. Something like that. I'm sure you can do your research as a writer and find some books that deal with this kind of stuff. If not, make the books up. People will suspend their disbelief when they go and watch a movie.
So now let's take a look at the story we have.
We have a serial killer, possibly on death row. His jail cell is cluttered with books on voodoo and dark magic. Okay, now we're getting somewhere.
So let's go to step two. If this person is already in a jail cell and has the title of a "Serial Killer" then my guess is that somebody isn't going to come in and interrogate him. Unless there was a break in an old case, or something like that. Soooooo, let's make this person who comes into the jail cell a Priest. Maybe not only is this guy a Priest, but maybe he is the prison psychologist. Let's add one of those white collars on his neck and a patients file folder under his arm.
So now let's take another step back and see what we have now.
We have a "Serial Killer" ( let's add that he is about to be sentenced to death in the next week).... Okay.
So we have a serial killer who is on death row. His jail cell is cluttered with books on the dark arts, black magic and voodoo. The prison Priest/psychologist comes into the cell holding the serial killer's file. He comes in and greets the inmate. The inmate is chained up to a chair, (Dr. Hannibal Lecter style) and is kind of in a daze looking off to the side, not even paying attention.
The Priest/doctor tries to talk to the inmate. Perhaps you can sprinkle some exposition here since the inmate isn't being receptive to the conversation. Maybe the doctor can say something like, "You only have one more week left until you walk the hallway, is that what you want to happen!?"
The inmate, who looks like he is shot up with a tranquilizer dart, mutters something. The Preist/doctor can't quite make out what he is saying so he gets up out of his chair, and walks over closer to the inmate. The inmate mumbles something again but still, the Priest/Doctor can't hear. The priest/doctor gets really close now. SURPRISE!!!! The inmate springs up and grabs the Priest/doctor. His eyes roll in the back of his head and he mutters some kind of voodoo mumbo jumbo. The Priest/doctor yells for the guard. The guard comes in and cracks the serial killer with a night stick, knocking him out....
The Guard looks to the Priest/Doctor and says something along the lines of, "Doc, there's no use. You're never going to get into his head the way you want to".
The Priest/Doctor can reply with something like, "Yes, I suppose you're right".
The Priest/Doctor character leaves the cell with the guard. The guard locks up and returns to his post. The Priest/Doctor walks away down the hall.
As the Priest/Doctor character is walking away, he rips the white priest collar off of his shirt and throws it to the ground. His eyes turn white and he has a grin on his face that stretches from ear to ear...
THE END!
I dunno, something like that. I came up with this stuff as I typed it out and literally put zero thought into it. So if it sucks, don't hold it against me. But you can get an idea how to develop story, characters and the reasoning behind things.
Hope I helped a little bit, and if you have any questions, feel free to ask!
(Also, please excuse any spelling/grammar errors. I wrote this pretty quickly and didn't proof read!
)
Best,