I'm not sure specifically which 'Soloist Horror Story' you are referring to. There could be many independent or less - known horror stories with that title. Without more context, it's difficult to detail the cast.
Again, without more details about this 'Soloist Horror Story', it's impossible to say for sure. But in general, in horror stories, you might find actors who are good at portraying fear and suspense. They could be new and upcoming actors trying to make a name in the horror genre.
The Soloist is indeed based on a true story. The filmmakers took real-life events and transformed them into a cinematic narrative while maintaining the core essence of the original events.