During the course of Avatar: The Last Airbender Aang displays a different way to interact with his previous lives: by turning into them for a short while. He did this twice with Roku (once when confronting the Fire Sages, once when talking to Jeong Jeong) and once as Kyoshi (during the events of his trial in Chin Village). Not only that, but it also alters his Bending abilities. When Aang turned into Roku for the first time he managed to Firebend like a master, something that Aang had not even attempted at that point.
But what exactly is this ability? Is it some kind of Bending? Or is it something inherent to the powers of the Avatar, like being able to communicate with them? If I am not mistaken Korra never does this, instead communicating with her past lives a few times before the events of the end of Book 2.