This is something I've actually thought about a lot... I'll link a few of my previous asks that were relevant to this at the end, and get into some new info!

SO. I don't actually write "smut" in the traditional way, I use nsfw scenarios either for "humor" or as a backdrop for something else going on in the fic. So this guide will be focusing on the latter... (Ie I'm less interested in just going "here's them fucking" and more about what's going on in their minds. Idk how to write hot smut / pwp. I have tried... I have not succeeded.... mad kudos to the smut ficwriters)

I think there are a lot of themes to explore with them which might be a good place to start. Some potential ideas are "humanity vs doll" (could factor reciprocity / body image / general nudity), vulnerability (letting themselves be human / imperfect), corruption (like how Mika always infers that they're going to go to hell together / Shu being Mika's "mentor"), guilt (for how Shu has treated Mika before / fear of taking advantage of the other one because of their mental state), etc. I'm just rambling here and I actually don't think any of this would make for hot smut LMAO but it's just where my brain goes.....

Okay trying to talk about like. Actual nsfw situations. I think what helps me is just referencing canon when I can. I often use canon to interpret and justify things in a way that makes sense to me. For example, I'll talk about the dragonfruit story. Shu won't eat the dragonfruit because he says it looks like innards, showing he's a picky eater. I don't think Shu would immediately jump for joy at the idea of performing oral on Mika because of something like this. If he won't put dragonfruit in his mouth why would cock be on the menu? However, this was a story that took place a long time ago, due to Shu's character development this could be something he'd approach. Maybe, performing oral on someone is really special to Shu because he sees it as a sacrifice of comfort on his part so that his partner feels good... So you see how this can kind of be bent to go either way depending on how you take the information that exists?

"Would he behave like this? Would he say this? Would he do this?" and then just thinking of times in canon when it happened / implied to happen helps me a bit. Of course I'm not perfect and there's plenty of people who disagree with my character studies lol but I think focusing more on your own interpretation of the characters to keep it to what feels in character to you would work best.

That being said, here's my personal cheat sheet for Valk nsfw situations
1. Shu is at least verbally a prude- can go either way in a fic though for plot reasons (post-astraea) /mental state, or he can be a hypocrite (says one thing, does another. like. what is maintenance...)
2. Mika is a bit of a masochist, he also really likes being praised (though not without good reason otherwise he doesn't trust it) - S/M + degradation + praise are all on the table
3. Maintenance. Again, what is maintenance?!? Up to you :)
4. Shu is weird about thighs / has a preference for a smaller build + more androgynous look - can be fun to play with gender / transgender characters
5. Mika likes eroguro. Shu... respects this. (Shows that Shu is willing to accept the sides of Mika that don't align with his preferences, may be more willing to try weird stuff Mika's into because he loves him <3)
6. Mika doesn't want to get Shu "dirty" - I feel like he might have a hard time expressing desire to Shu in the beginning because he idolizes Shu too much.

There's more but I'm probably missing them. Hope this helps, I feel like I was a bit all over the place here feel free to send a followup if this sparks more discussion :)

https://retrospring.net/@itiaskia/a/112474991531350611 - How would Shu react to Mika being more "freaky" than him

https://retrospring.net/@itiaskia/a/112357070014230261 - shu's prudeness vs mika's preference for the grotesque

https://retrospring.net/@itiaskia/a/112662191090733593 - shu's prudeness in relation to asexuality

https://retrospring.net/@itiaskia/a/112474907641037723 - mika's masochism