Curate, connect, and discover
Headcanon: Marc has claustrophobia.
It makes sense, given how Randall died, so Marc would definitely have PTSD from that which was definitely a contributor to his DID in the first place.
Naturally, ‘When danger is near, Steven Grant has no fear’ Steven isn’t claustrophobic and so he doesn’t at first realize that his ‘fearless’ host is in fact scared of small, dark spaces. Especially underground. Mainly caves. With water.
I also recon that this is a big reason why Marc didn’t insist on fronting in episode 4 until he absolutely had to, he was quite happy to let Steven do all the dark, underground tomb-raiding, something he probably carefully got out of doing when he had been out doing archeological stuff with Layla in the past.
(This is, if she doesn’t already know about his claustrophobia or know about Randall, which is definitely the kind of thing that would be on The List of Marc’s Secrets From Layla.)
But I feel like there could be a time when for whatever reason, Marc and Layla have to go into a cave, there could be a river in there too, it could keep getting louder as they go deeper in and the tunnels get narrower, and Marc is pushing down panic but it’s getting harder. Steven, from the reflection in the water, would figure it out pretty quickly and gently try to convince Marc to let him take the body, but Marc keeps insisting that he’s fine. He’s not, and Layla notices, she and Steven trying their hardest to talk Marc out of a panic attack.
Then Marc goes very still, eyes flickering, then lets out a deep breath and Steven turns to a confused and concerned Layla just to say ‘let’s keep going,’ then he walks off.
Later, Layla would find time to ask one of them about what happened back in the cave, and that could be how Layla finds out about Randall, Wendy, and how Marc created Steven in the first place.
Just an idea.