So I stumbled across these two sentences
when you are low-key dead on the inside ...
They look low-key dead inside
What's the difference between inside and on the inside? I don't seem to find the difference at all.
Including the prepositional element (on the) could be said to impart "emphasis", but I think it's only really appropriate in contexts where inside is being (more or less explicitly) contrasted with outside.
That's to say, He felt dead inside has no real implications for how he felt "outside" (which isn't even a very meaningful thing to say anyway), but He felt dead on the inside would normally only occur in a context where on the outside (his external appearance) there was no evidence of this.
Apart from that there's no real difference, but this NGram should make it clear that native speakers usually prefer the simpler (prepositionless) form.