I remember my mother, who was 9 years younger than her nearest sibling, and therefore an "only child" in many ways telling us she had imaginary friends named Buckem and Mean-Boy. I don't know if she encouraged us to have IF, or if it was natural. I remember one of them wasn't nice, but the others were. So it gave us the opportunity to role-play social situations. (my very much adult and too-serious analysis) I remember my siblings (I'm the middle child) an I played out elaborate story-lines with our toys and figures. We named generic figures and dolls also.

I’ve talked to myself for as long as I can remember. For me, it’s always helped me work through complex problems out loud since just thinking about a thing has a tendency to spin me into wild tangents.  It also calms anxiety, orders my thoughts and I often make myself laugh!  For the longest time I thought there was something wrong with me, especially when I was young.  Again, this was the 70s so crazed serial killers headlined by Charles Manson were in the public consciousness.  I thought I might be leaning toward schizophrenia as that almost always manifests itself at a young age.  As I got older and still did it, long past the age where it would actually be a mental problem, I was just embarrassed.  It wasn’t until the last 15 years or so where I’ve accepted it as part of who I am and how I process life.  I’ve also read that it’s fairly common, just a thing that most don’t discuss.  Although I’m good at curbing it in public situations, I am thankful that talking to myself alone in my car no longer draws stares of confusion.  People just think I’m on the phone!   

Definitely has some real arthouse horror vibes, now that you mention it. Would have to be a period piece to really nail that genre...

Darn you, Matt! You always got to be Batman, then. I had an older brother and I was always stuck with Robin. Grrr...!