La Linea (or "The Line") is a minimalistic cartoon set in a world which mostly consists of a line. It comes to life by being shaped as a person who moves along the one dimension of its world. Most of the events that make up the plot of the episodes come from obstacles added to that world; the obstacles are built into the very line which also is the material of the main character, but they are deliberately played with by a hand (presumably that of the artist). Sometimes the hand removes the ground on which the character can move; sometimes it provides the character with helpful devices, or simply fills in holes in the floor. At one point, the hand helpfully pushes an elevator a few inches in order to close up with the floor and allow the character to move on (at around 1:15 in the video).
Now I've suggested that worlds of fiction are characterized by metaphysical apartness, and this one is no exception. This means that we could no be part of that world, that we'd be no actor within it; we're limited to observing it.
But if that is so, then what about the hand? Isn't this a case where the fictional world (the world of the line and its inhabitant) and the real world (our world, where we, among other things, can use our hands to move things around) connect and interact, where they aren't separated any more? Is not, in other words, this real/fictional-world couple a counterexample to what I've claimed? What we seem to see here is a connection, a direct interaction (going both ways) between the real world and the fictional line world.
I don't think so, and the reason has to do with where exactly the borders between the imagined world and reality run. The interaction between the hand and the line character is not a case of reality-fiction interaction because both of its elements are part of the fictional world. The hand is just as much a character in that world as the line person is. You can make the test: your hand couldn't do the things this hand can do. Not even the artist's hand could do that: what you can see is not a physical interaction between the hand and the line, it's just an impression to that effect created by a technical trick. (Which is of course a common thing in film and television; the point here is just to show that all of it firmly belongs into the fictional world.)
So the fictional world comprises a little more than it looked like at the beginning; but once we've sorted this out, there is really no more question about metaphysical apartness holding between the real world and this particular sort of fictional world. Again: those two are separated, in such a way that we, as inhabitants of the real world, couldn't be part of the fictional world, we couldn't be inside it. And that is not just a separation in practice (such as when you are watching what happens on the other side of the street, but can't do anything about whatever you observe). It's a separation in principle: you, as a part of the real world, couldn't possibly be inside that fictional world (in the case of looking over the street, you might have been over there, you could have crossed the street a couple of minutes ago, and then you would have been part of what's going on over there right now — but there's no such thing that you could have done in order to become part of the line/hand world). (Sidenote: that you can't be part of it doesn't mean that such a world doesn't share some structure with the real world. In fact, every fictional world has a lot in common with the real world. In the case of La Linea, there is still time structure, some sort of gravity, energetic Italian exclamations, and the like, which is all at least similar to the real world. However, for all that, you can't be in that world.)
In summary: this nice little example gives us not only another, slightly different sort of imaginary world (an imaginative one, to be sure) to test the criterion of metaphysical apartness as a marker of unreality; it also helps to show the importance of looking closely where the borders between fiction and reality run, and distinguishing carefully what's part of reality and what is still unreal.
 Being able to exactly pin down where the lines run between fiction and reality is very important; if one doesn't, one might very well end up with shrill, though interesting-sounding, theories about self-interpreting texts and multi-level fiction/reality fusions which really have no other basis than someone's not looking carefully. I've criticized that sort of line-blurring in the research literature on Kafka's Über Gleichnisse in my BA's thesis.