OK. So here's the idea. You and your palls can enter moving pictures (think Last Action Hero or that recent Dr. Who episode where the circus guys came through film).

That opens up more genres than you can shake a stick at (and probably more work too!).

What do you do? Why do you do it? Are there some wierd organisations involved? Can things on film come out into the real world? Are all films subject to this? What about the actors involved, are some of them a wierd invader? Aliens? Are some murders carried out by folks coming out of TV sets? Cinema screens? Can King Kong rampage New York at the screen premier? Is Alien vs Predator a real thing once the print is made? Are they as shit when they emmerge? (if they can). How many people know the dangers and how frequent are the dangers? Do screen characters have a life outside of the film yet are still contained within that world? Is there more to that world than shown on camera? What would the real life actors have to say about that? Are they ever confronted by their celluloid doppelgangers and what happens if they are?

How do you detect which film is a danger and who within it? How do you enter the movie/programme and how do you get out again? Can this danger appear at any time and screening or have you cleared the danger once you have entered the film? Not every screening presents a danger surely?

How do you know what is real and what isn't? Maybe your 'real life' is contained within a moving picture and you just don't know it.

Do you appear on the screen when you enter it? Are your actions shown? Can you change the movie via your actions to thwart your foes? How would an audience react if you did? Perhaps you live 'offscreen' instead and interact via the offscreen world of the movie/programme.

Any thoughts, shout-downs or comments?

Too many options. I think the reason this was left out was you didn't define enough.

In some ways, this is Donaldson's Mordant's series: The Mirror of Her Dreams (1986) and A Man Rides Through (1987) .   Or really, it is Amber using video rather than cards.

So my take on this:

Films create a link to alternate worlds/universes based upon their stories.  The screen creates a floating portal between them. They take you to that point in time in that universe.

So if you enter Star Wars IV, and miss leaving the universe by the end of the movie, you can hang out until SW V come out and portals reappear.

PCs can "shift" into them and out of the, as well as bring people and things in and out of them.  (So I need some guns, I go to DVD cue up the Neo Arming scene in The Matrix, excuse myself.. get some guns.. and come back to reality).   Maybe those with "the power" can, with enough time and effort, change where a portal is (in either space or time).

Either some of the being in those alternate worlds can shift through the floating portals. (I would have deamons/ unmaterial bad guys using film side being).  

Those that can shift might have a feel for the shifting. Thus you could "feel" when danger was comming.

I assume that nobody knows about this and those that do are trying to supress the knowledge of it, otherwise nobody would ever be watching a film or tv again.

I would assume your actions would only change the screen you entered by (and any screen you plan on exiting from).

Now the question really isn't the mechanics. The question should be, what kind of stories do you want to tell with this?

Do you take the Alternate History take, when you are keeping fixing any changes the bad guys make?

Do you just want to make it a sandbox and let the PCs run amok (and amok and amok, if you have the Witches of Eastwick)?

Are aliens from another world utilizing these alternate worlds to enter our world and invade/ satisfy their needs.. using being from the movies?