Translated from grok via @/phaidei_taro on Twitter/X’s thread.

• At first, I auditioned for Baek-bal-gwi (the white hair Oni from Acheron's world). After recording Baek-bal-gwi, the PD said, 'It will probably come out as a different character, see you then^^'
  
• When it was about to be forgotten, I got a call saying, “You have to do a parallel world character of the Baek-bal-gwi character I think you need to take another test or audition,' so I went to check it out.  
• So, thinking it’s a parallel world, it should be similar, right? I thought I could just lower my voice and make it deep and heavy, but when I looked at the setting, the character looked way too dazzling, like they’ve never done a single hard day’s work, a total pretty boy, a model student, and no joke, I was like, 'Oh, this isn’t my thing.'  That’s what I thought, so I went to the audition with a relaxed mindset. But then I got cast as Phainon. When it happened, I felt happy but also thought, ‘Uh, what do I do…?’
• Since I had to record up to version 3.7 without knowing what would happen to Phainon  later, and I didn’t know exactly how the story would unfold, I looked up the original character, Kevin. I analyze Kevin and realize, "This is how it changes, this is how it goes," and I roughly guess that Phainon might end up like this too.  
• During the PV, the screaming part was recorded after the story was completely finished, but since the dialogue overlapped in parts, we recorded the dialogue first and then separately captured the breathing for scenes like the fighting ones.
• When recording breathing, Phainon moves so intensely, so I thought it might be good to put some emotion into, whether we use it or not. So, I added some screaming ad-libs. But I didn’t know it would end up being used in the PV.  
• Something similar happens in Phainon’s anime video where there’s a part in the original music with an “Ahaha-!” (the part where they’re laughing maniacally with their shirt off). I also added laughter here, but it ended up being removed in the final version
• *In the middle of the video, while talking while watching the Phainon animation, briefly acting out the part where he's laughing in a state of madness (the part that was skipped during recording). But it's so good because it really feels like a person who's completely insane with madness at MAX, I highly recommend listening to it) (1:37:50)
  
• TMI: [Korean Phainon VA’s] throat wasn't in good condition when recording battle voices, so it took 2 hours to record

• Q: Was it difficult to with various emotions and dramatic emotional changes?  
  A: It was very difficult (crying) I acted with the knowledge that Phainon would later turn dark. At first, I set the acting tone to feel heavy and like an exemplary young man (for the Japanese voice acting). During the directing process, the PD initially told me to make the tone younger and to capture the feeling of a handsome, charming guy. It was said that they would go dark later, so early part was deliberately made typically bright in complete contrast to that. In terms of content, the character is actually good-looking, but personality-wise, they weren’t particularly charming or anything; rather, it was more like, “Why is this guy so annoying? So boring, such a buzzkill.”  They were only fun when they appeared with Mydei lol. As it progresses backward, there are more parts where the emotions deepen, so in the latter part, since these sections are important, I pushed forward by saying 'Even if the voice sounds aged in places like this, please understand (it probably means doing it with a deep, resonant tone)'By the time of the speech, I conveyed 'I'll go with emotions as the focus,' but the PD pushed back saying please put in even more emotion! So, I ended up expressing emotions quite intensely, but when it came out, it felt even more dramatic. I also thought about whether people watching this would be able to enjoy it content-wise, and since there are parts with a lot of monologues, I was careful because it could easily become boring if not done right, so I put a lot of effort into the acting at those moments. (It seems to be about 3.4)
• Q: What was the most memorable scene while acting?  
  A: The final part of Phainon, the part where the journey is completed. I prepared for about two weeks, and the line "I’ll bring down the morning star, the dawn’s light!" was delivered with such intensity! It left a strong impression lol. The part where I exploded with emotion, like "It won’t go your way!" is the one that stands out the most in my memory.
• Q: Regarding the Flamereaver, did you know their identity in advance?

A: I thought Phainon was suspecting that something would happen later, but the Flamereaver is pretending? Is this the one who becomes it when Phainon turns dark later? When I asked, the PD said yes. But this character doesn't speak well, heh. Always like "fire spark..." and so on, so somehow this character seems like a fallen villain. I thought about adding a rough vibe, but it didn’t come out as much as I expected.

• Q: How was it acting as Ciphainon?  

A: When I saw Ciphainon written on the cast list, I was like, what’s this? lol  I tried to mimic Cipher’s way of speaking as much as possible after hearing about it, but if I stuck too closely to Ciphainon’s speech style, it felt less interesting… So, the PD asked me to make it a bit more spirited and fun, so I went for a more comedic performance. There was even some bleeped-out swearing, and the PD actively directed me to do it like that! I actually swore during the recording, lol. Personally, I was a bit disappointed because my throat wasn’t in great condition when I recorded. I should’ve done it with a bit more clarity…