(It's a mutual effect. Katsuki is concentrating on the correct amount of freckles on Izuku's face and he can surmise that if anyone tried to counterfeit a Deku, they would probably fuck up on that part. It's the easiest, clearest memory he has and maybe it means they knew each other before in their supposed lives or maybe it means they just agreed to do this together. Whatever the case, the only thing that makes sense is that they wound up here together.

Scars. Freckles. Dumb hair. Annoying wobbly voice.)

Clowns are for toddlers and weird...(He pauses, frowning.)

American horror movies. (He forgot the word 'American' for a second there.)

Yeah, and you're Deku. (A pause, his tongue curling in his mouth.) Izuku.

(He follows Izuku's glance.)

Looks like it. (He moves to his feet then, but he doesn't let go of Izuku. He just winds up moving to grab his hand instead and pulls at his arm to join him.)

But it's easy to fake pictures these days.