This is the main departure from GTA V, where Michael, Franklin and Trevor could be swapped almost at will in free roam. GTA VI gates switching behind narrative logic: you can change character when the story allows it and not when it does not.
In practice that means three situations. Some missions lock you to a single protagonist, some switch you at a scripted moment, and some leave both available so you can move between them freely.
The August 2026 presentation showed switching happening mid-action with no loading break — moving from Lucia driving to Jason leaning out of the window to shoot, and back again. It also showed switches during ordinary moments, such as one character scrolling their phone in the passenger seat.
The sequence where Jason and Lucia come under attack near the end of the presentation was shown as free-swap, confirming that combat encounters can let you move between both rather than locking you down.
Rockstar has described the pair as able to work as a couple or operate independently, so expect sections where they are separated and pursuing different objectives as well as sections where they are side by side.
Both are playable across the whole story rather than being divided into chapters. How their relationship develops is also shaped by play, though Rockstar has not detailed how far that extends.