How is it determined which portraits in Harry Potter can talk?

Question

How is it determined which portraits in Harry Potter can talk? Because if I had a loved one die - like Sirius - it would make me feel so much better if I could talk to them. So how is it decided which ones can talk?

Answer 1

The author answered this at the Edinburgh Book Festival on 15 August 2004:

All the paintings we have seen at Hogwarts are of dead people. They seem to be living through their portraits. How is this so? If there was a painting of Harry’s parents, would he be able to obtain advice from them?

That is a very good question. They are all of dead people; they are not as fully realised as ghosts, as you have probably noticed. The place where you see them really talk is in Dumbledore’s office, primarily; the idea is that the previous headmasters and headmistresses leave behind a faint imprint of themselves. They leave their aura, almost, in the office and they can give some counsel to the present occupant, but it is not like being a ghost. They repeat catchphrases, almost. The portrait of Sirius’ mother is not a very 3D personality; she is not very fully realised. She repeats catchphrases that she had when she was alive. If Harry had a portrait of his parents it would not help him a great deal. If he could meet them as ghosts, that would be a much more meaningful interaction, but as Nick explained at the end of Phoenix—I am straying into dangerous territory, but I think you probably know what he explained—there are some people who would not come back as ghosts because they are unafraid, or less afraid, of death.

So it seems that only the portraits in Dumbledore's office can really talk like shadows of their live selves, and not just because they're portraits but because they were headmasters/headmistresses and their imprint somehow remains in that particular office. Making a portrait of Sirius wouldn't somehow bring him back, in the same way as there's no meaningful interaction with Harry's parents through portraits. The only reason Dumbledore is able to communicate with Snape through his portrait after his death, or Phineas Nigellus Black etc., is because they're old headmasters communicating with the current headmaster in the headmaster's office.

(The same quote is also used at Can Hogwarts headmasters only visit one other painting of themselves, and whence cometh it? and How can the portrait in the Muggle Prime Minister's office talk as per JKR's definition? but I didn't quite consider your question a duplicate.)