Rockstar's Head of Development, Rob Nelson, explained that in the open world, players will have greater freedom, but in story missions, character switching will depend on the script.
The developers have divided missions into several categories. In some, players will be able to freely change characters, choosing a suitable playstyle. In others, switching will occur at a strictly defined moment. For example, during a getaway after a bank robbery, Jason first shoots at police from the passenger seat, after which the player takes control of the car as Lucia.
Rockstar will also intentionally block character switching in scenes that require a perspective from only one side. An early mission in a residential high-rise was cited as an example. Ultimately, control was assigned to Jason due to his military background and special forces experience. Handing control to Lucia at that stage would have seemed unnatural.