The best path into Ethan x reader scenes is often recognizing what distinguishes him as a character—his intellectual curiosity, his guarded nature, his mix of vulnerability and unsettling analysis. That tension is fertile ground. A scene feels true when the 'reader' character engages with that intellect, not just in a 'oh you’re so smart' way, but by pushing back, asking questions that challenge his theories or reveal his blind spots.
Romance for him probably isn't grand declarations. It's in the quiet, intense focus he directs at someone who sees past the 'movie-buff' persona to the person underneath. Maybe a scene where the reader is hurt or scared, and his response isn't just comfort but a meticulous, almost clinical assessment of the threat, which then falters because his hands are shaking. The disconnect between his analytical mind and his emotional reaction creates a compelling moment.
I'd avoid making the reader a passive recipient of his quirks. Give them their own expertise—maybe they're practical where he's theoretical, emotionally perceptive where he's closed off. The spark comes from friction and mutual fascination, not just admiration.