The thought of a cinematic mode puts me in mind of a one-off session meant to be played at a convention or as a break from a regular campaign. Is that about right?
Yes and no.
The main characteristics of a cinematic adventure are that:
1) It has to be played with pregenerated characters, because they are made to tailor the story.
2) Linked to that, each character has a Hidden Agenda, a personal goal that forces the player to aim at an objective during the adventure. It doesn't have to be mean, it could be "make everything you can so that the whole crew survives". But that would imply, for example, that the player should put his character in danger to protect others. As the scenario might have different Acts (based on dramatic twists of the plot), the Agenda could change for each character with each Act.
3) The cinematic adventure is thought to be very mortal, so by design NPCs are planned during the story so they can become replacement PCs.
But a cinematic scenario can be played during more than one session. Chariots of the Gods is played by many people in two or even three sessions. And the last one released, Destroyer of Worlds, is designed to be played over 3 or 4 sessions.
Of course, it is a very adequate kind of scenario to be played also in one-shots (like Hadley's Hope, the one that comes with the Core Book), but it's not a requisite of cinematic scenarios.
Also, as you point out, they can be used as side- or background stories during longer campaigns; like the typical "you find a recording of what happened in this derelict base before it all went to hell. You press Play on the player, aaaaaand... here you have your new character sheets. Choose your new character for this scenario and let's see what is in that tape and how the whole settlement became extint in one night...".
