Sorry it took me a while to get back to you, I’ve been dealing with back pain all weekend and have been in bed most of that time. 

It’s not confusing at all, I totally get what you mean and thank you for asking, I’m flattered you think I’m a good source of information for something like this since I, too, struggle with getting “my” voice out of their mouths lol. 

First things first, something like this takes practice and patience. Writing something, reading it over, visualizing them saying what you wrote, and adjusting as needed. 

Consider their word choices. It’s a little challenging sometimes because as with most anime we’re watching a translation (I’m presuming, unless you can understand the native tongue it was broadcast in) but assuming you’re watching subtitles you’re getting the gist of their word choice. 

Such as, Midorima and Akashi speak more “proper” this conveys their intelligence, self control, seriousness, power, etc. 

Aomine and Murasakibara are more casual speakers, they’d be more likely to use slang or sharp speak. 

Kise is a bit harder because there are two schools of thought: anime version and manga version. I’ve never read the manga but I have read about his characterization in it and he’s…less bubbly and more cold. But you can do both to show his dual nature. It actually can make him an even more interesting character and not just “the dumb blonde”. 

Think about how they communicate in terms of tone inflection, pace, facial expressions, gestures. All of these things help bring them to life on the page. 

You don’t have to necessarily do all of this while you’re writing the scene. Get your ideas out on the page so you don’t lose them and then go back and tone it properly for the characters. 

I hope that makes sense and is helpful, please feel free to ask more questions or for clarification if needed and thanks again for flattering me by asking MY advice on character writing!