I'll admit I usually find character casting calls kind of tricky. I do like to have a muse in mind while I'm planning out the story and building an inspiration board. I don't actually write a character AS a particular actor or model, but it does help to get a vibe going from looking at different images. That being said, I don't always narrow in on a particular person as a muse, since it's hard to find someone who 100% matches your vision.
However, the muse I used for Ace Turner comes pretty close. A picture of him popped up on my Pinterest, and my reaction was something along the lines of, "Yoooo! It's Ace!" They even have similar bird tattoos. The guy's name is Thomas Davenport.
Dove Cameron also comes quite close to my vision for Stéphanie. I think they've got the same mix of sunshine and danger going on. Admittedly, I've never actually seen anything she's acted in, but the aesthetics are spot on.
You can find more images I used for character inspiration, as well as story and setting details, on the book's official inspiration board. Of course, if you haven't met Ace and Stéphanie yet, head on over here to get on that!