Thinking about Demon Form, I think I have a theory. Basically, the stronger and more emotional you are, the more access & control you have to & over it. That’s why weak Sinners never show it, even Overlords we see on screen don’t. Vox, however, was able to slightly go into his partial form when really excited during Don’t You Forget reprise. Weakened Alastor (strongest Sinner, a lot stronger than Overlords) can tap into his Partial Demon Form fairly easily (though he was a bit excited to finally put Vox into his place), and 100% Vox (even stronger) went into his Full Demon Form in fierce of range, and can be in his Partial Demon Form more easier. He lost his Demon Form after his popularity dropped too, by the way. When he was at lowest popularity, he wasn’t able to go into Demon Form despite having the most emotional breakdown he could. Charlie can go into her Partial Demon Form at will and was shown to transform fully when enraged (although she is only half a Sinner and a bit of demon, but still has some Аngel part to her). Full Power Alastor can go into his both Partial and Full Demon Forms at will. Lucifer can casually tap into his Demon Form at will whenever (although he isn’t fully a demon).
Whaddya all think?