When making characters (and immersing myself in them), I write up little details about them. I try to get to know my characters some more. Like, does ABC get along with their mother? Why not? Would there be a chance of reconciliation? What is XYZ's "happy place"? How did they pick this place? What do they do here? etc. I fall into a rabbit hole of things like that. The details could be as deep as those or really inconsequential like what their favorite cereal is or something. Basically, I often ask myself "why this? why that? how even?". Then I try to connect the dots.

If this doesn't work, I look up things that would remind me of the character. This could be past writings, my notes about the character, aesthetic photos, music, outfits, just whatever. Tumblr, Pinterest, and TVTropes are particularly helpful when it comes to this.