Presumably in the case of George R. R. Martin and a Song of Ice and Fire, one of the reasons for slow progress on the next novel in the series is that a reasona

Presumably in the case of George R. R. Martin and a Song of Ice and Fire, one of the reasons for slow progress on the next novel in the series is that a reasonable ending was always known, but each character's arc has changed so much from what was originally planned, that reconciling the two is very difficult. For older authors like Charles Dickens, who wrote and published many of their novels chapter by chapter in magazines, plans would presumably change based on audience response.