Captain America: The Winter Solider showed us just how far Steve Rodgers will go to save his friend, but according to Anthony Russo, we haven't seen anything yet.
"The relationship between Steve and Bucky is one of my brother's and my favorite aspects of the Captain America movies. It's so complex and tragic. They have a devotion to one another that is undermined and devastated by fate, and exploring how they endure that is fascinating for us as storytellers and fans. We feel Civil War is the best chapter yet in the story of their relationship. The character has gone from being Bucky Barnes to being the Winter Soldier, and now in this movie he is something else. We explore whether he and Steve can ever have a relationship again and, if so, what that relationship can be."
From snippets in the trailer, Bucky is seen in another containment vessel of some kind, which indicates maybe he isn't in complete control of his actions yet. If that is the case, it could be the thing that creates the divide between Steve and Tony, resulting in the proposed government oversight. He and Bucky's friendship is something Steve will go to any length to protect, and in Civil War we will see how far that really is.