Thinking about Billy crying and how, really, that's his first gut response to being hurt or afraid. Canonically, Billy is a crier. As a child, he cries when he's on the phone with his mom after she left him. He cries when Neil is yelling at him and calling him derogatory language because he's afraid of getting hurt. He holds back tears when Neil abuses him for not knowing Max's whereabouts and then finally lets the tears fall once Neil leaves the room. His eyes are a bit glassy with tears after Max injects him with some unknown drug (read more here). Sometimes, when he yells at people, there's this crying crack in his voice. He cries in front of Max during the sauna test. He's crying on numerous occasions when the Mind Flayer makes him do things that he doesn't want to do. He's crying alone in his room before Jane comes to look into his memories. He cries when he's about to die, and Max is leaning over his body. And however you want to interpret that S4 scene in "Dear Billy," even Billy's memory cries over the things Vecna is making him say to Max. All of this is Billy crying around pain (whether physical pain or emotional/mental pain) and fear.
Now, Billy is somewhat able to mask this by leaning into anger, which I think is absolutely a fear and preservation response to the fact that Neil abuses Billy and regularly degrades him with homophobic language that all center on pointing out that Billy's femininity is weak (and therefore, by extension, crying is weak-- you can read more about Billy, crying, and gender non-conformity here). I do believe that if Billy didn't cover up the fact that he wants to cry with anger, we would be seeing him cry a lot more. Which also goes to say that in the canon-divergent world in which I write Billy surviving the Mind Flayer and getting out of Neil's grip to be in a space where he can heal, he's going to cry more often as part of his healing process from years of trauma and grappling with the persona versus the self.
But a point I want to make here is that Billy still cries in the series, even if Neil tries to beat the sensitivity and emotion out of him. He still cries. And when Billy cries, it's the crack in the persona he emulates to survive abuse. And this makes me think back to Dacre's point that in S3, he acted with intention that Billy was there in the eyes while the Mind Flayer was there in the body. Which I think we can extend that further into Billy's characterization as a whole, not just the Mind Flayer plot. That Billy crying is a reveal of the self behind that loud persona that exists as a protective shield.
