Hi Eulalie,
the way I get it, Hades could take people any time, but doesn't- in fact he accepts people inside his halls at the time that they have been fated to die. During the Trojan war the ghost of Patroclos appeared to Achilles and tells him, among other things, that he died when he had been fated to, and that Achilles also was fated to die under the walls of Troy. It's from that incident that I get this basic picture. But during the same war, Sarpedon, a son of the divine Zeus, and dear to him, was also fated to die. Now the story is that Zeus said to his wife that he had a mind to save Sarpedon, but she pointed out to him that he could do it if he wanted, but that if he did, then many of the other gods who had children in the battle would want to save them too, and that he will cause much resentment in this way. Well if that's all true, then Zeus could go against fate if he wanted to, but chooses not to. Perhaps Hades operates in the same way- that is, he could take people on a whim if he wanted to, but doesn't.
How does this match with the film though?
Best wishes.