Department of Queries: Portraits

Here is the question that we will explore for the DoQ: What dictates the actions of portraits?
Calie and I had a great discussion over this, including different aspects of portraits, including travelling, communication, purpose, and the 'portrait world'.
We began by talking about what we think the portraits would see on their side of the frame. Would it be like another world back there, where everything is three-dimesional? Would it be two-dimensional since they are painted? Do the same rules apply to the objects if they are present? I think that first off, the frame is like a window to real people; that even if all we see is the person from the waist up, they would still have a body, it's just not shown. Otherwise, how would they travel to their other portrait? I think that the 'world' behind the frame would be three-dimensional to them, and so would the objects. I think that this is proven when the Fat Lady and Violet drink all of the wine in the portrait of the monks.
Another thing to do with backgrounds was the space between the frames (or the walls their hung on) in their background. We both agreed that there would have to be some kind of space between them, but we were stuck on how they would actually move into other portraits. We see in many instances where the portraits visit others. Would there be something in the 'space' between them, like a ladder or stairs, to get to ones higher or lower than them? That's a question that Calie had. I had always imagined something like a hallway on the sides of their frame, where they could choose which way to go, and also a path to go to their other portrait. But as for traveling to their other portrait, it says that they just walk out the side of their frame to go to their other portrait, but when they do, they don't appear in their adjacent frame. We couldn't agree on whether or not the distance between frames on their side was the same as on our side, such as just the length of wall between them, or a simple threshold.
Something we found intriguing was Ariana's portrait. It seems to break a lot of portrait rules. Firstly, all of the portraits in the Headmaster's study that we see travel walk out of the side of their portrait to appear at their other portrait. When Ariana visits her other portrait, she "turned and walked away, not as people in portraits normally did, out of the sides of their frames, but along what seemed to be a long tunnel behind her." Another very odd thing is when Harry asks Phineas Nigellus to bring Dumbledore into his frame to talk to them, he says: "The portraits of Hogwarts may commune with each other, but they cannot travel outside the castle except to visit a painting of themselves hanging elsewhere. Dumbledore cannot come here with me." Maybe this only applies to Hogwarts portraits, but when Ariana goes along the path of her portrait, she is able to bring Neville through. He is also is seen in her portrait when they are travelling back. I don't think he actually travelled through it, since "the [portrait] swung forward on the wall like a little door, and the entrance to a real tunnel was revealed. And out...clambered the real Neville." We know that portraits can be guarding entrances, but how is it that Neville could be shown in the picture when he is inside the tunnel? We never see anyone in the fruit bowl or Fat Lady portraits when they travel.
That is it for this installment of Department of Queries. If you would like to add your opinion or critique, we encourage you to comment and let us know what you think about this query.