"Captain America: The Winter Soldier" — as great as it was — ends with one of its major storylines unresolved. What becomes of Bucky Barnes, now the Winter Soldier, after Cap shakes something loose from his brainwashed head?

Anthony and Joe Russo, who directed "Winter Soldier," are picking the story back up with Captain America's third solo movie, set for release in 2016, and they talked with me about why that storyline is what's really interested them about the further adventures of Steve Rogers.

While the specifics of the plot are not public knowledge yet — Christopher Markus and Stephen McFeely just turned in their first draft last week — there are a set of questions that the Russos hope will drive the story forward.

"Is the Winter Soldier the world's most dangerous assassin that's ever lived or the world's longest serving P.O.W.?" Anthony Russo asked. "Is he responsible for his actions since he was turned into the Winter Soldier, or is he innocent by reason of insanity? Where does this character live now? Is he ever going to be acceptable again to Cap in the way he once was, before he was the Winter Soldier? Those are the really complicated relationship questions and philosophical questions and emotional questions that intrigued us moving forward."

These questions might not reveal who the villain will be or whether the Winter Soldier will become Captain America, but it gives a sense of where the emotional heart of the movie will lie, an element that's been so important for the first two Cap movies.

"We spent a lot of time thinking about that relationship, between Cap and the Winter Soldier," Russo said. "How that processes through a filter of Cap's political associations, the specifics of that relationship, we pour it all in a shaker and mix it up."

"Captain America 3" will open in theaters on May 6, 2016.