About this scene
Commuter train, mid-evening, half-full carriage. She's in a window seat. Her friend is in the seat across the aisle, one row back. He's seated next to her. About a minute in she leans across and the BJ starts. Her friend keeps her eyes locked on her phone for the duration, scrolling without reading. The friend is acting normal in the obvious way. The carriage gets one new passenger at the next stop and he sits at the far end. She does not stop.
What to watch for
The friend is the second story. Watch her eyes through the reflection in the window across the aisle. They flick over twice in the first minute, then she commits to looking at her phone for the rest of the ride. The phone screen does not change. She is not actually scrolling. The new passenger getting on at the next stop walks the length of the carriage to choose a seat at the opposite end. He clocks the situation in his peripheral vision and keeps walking. Nobody says anything.