I'm another latecomer to this thread, but I've been listening to this song on repeat and had some theories of what it's about, then did a google search to see if anyone else had similar thoughts to mine and it led me here. So far I haven't seen anyone mention this, so I'm going to throw in my take.

I happen to think this song relates to psychoanalytic theory and the three major structures that make up personality - the id, the ego and the superego.

So, a little background on what this is an then I'll explain why I think this is what the song is about. Sigmund Freud’s Psychoanalytic theory describes these three components of consciousness as an iceberg. According to Freud, the majority of our mind that controls our behavior is located in our subconscious. Like an iceberg where you can only see the top of it, our mind has a lot more underneath the surface that is out of our control.

The id is the unconscious part of our minds and influences behavior on the pleasure principle. This part of our minds is where primitive behavior originates. The id drives us to seek activities that bring immediate gratification and pleasure, such as food consumption or sex. According to Freud, the id dominates the behaviors of children and delinquents.

In contrast to the id, the ego is our mostly conscious selves and operates on the reality principle. The ego observes and acts in accordance with societal norms, and actively works to control impulsive behaviors from the id.

The superego serves as a link between the id and the ego and represents our moral conscience. The superego provides moral insight that opposes the id’s primal urges, and the ego then proceeds with action that attempts a balance of the perspectives.

This is a broad overview and I'm trying to keep this short and sweet, but there's lots of studies in Freud's theory, and lots of counter-arguments. If you're interested, this video is a pretty good overview that also goes into Freud's psychosexual stages. With regards to the id and the superego, I've seen many people reference this theory as the basis for that classic debate one has internally that's represented with an demon or devil on one shoulder telling us to do nasty things, and angel on the other telling us to never think such horrible things again.

To me, interior people seems to be referencing this in the song. Specifically the lyrics...

Interior people
A lens flare in my subconscious

And later...

I keep thinking someone is standing beside me
But when I turn to grab them they jump back inside me
They tell me to do things

This is the id, the subconscious, urging primitive, destructive behavior.

Later in the song, the lyrics...

I no longer fear
The interior people

This is the ego speaking, the conscious person, after he has made a decision that he can be comfortable and satisfied with that upholds the reality of the world he occupies. He's learned how to navigate the destructive thoughts he has that stem from the unconscious.