This has been up for most of Thursday, but I was waiting to see whether or not it was an April Fools joke. Apparently, it is not. Heat Vision (and now everyone else) reports that Sebastian Stan - apparently one of the also-rans for Captain America - got a consolation prize when the role ultimately went to Chris Evans: He gets to play Cap's sidekick, Bucky Barnes.
In this case, the surprise is that Bucky is in the movie AT ALL, I'd say. Honestly didn't expect that. If you had to ask me what the first thing you'd jettison (or at least trim way, way down) for an Avengers lead-in Captain America movie, I'd say Bucky right off the bat.
Quick primer for non-fans: Since Captain America originated in the 1940s, he came completely with the then-all-but-mandatory "boy sidekick," a'la Robin. So goes the story, Stan Lee was never terribly fond of the boy sidekick angle, which is why basically none of the "Silver Age" Marvel Heroes started out with one. So when it came time to ressurect Cap - and concoct an altered backstory to erase the ill-advised 50s "Commie Smasher" Cap/Bucky books - Stan etc. opted to kill two birds with one stone: In the "new" version of Marvel history, the same circumstances that led to Cap being frozen in ice since the end of the war ALSO flat-out KILLED Bucky Barnes.
In the short term, "guilt at responsibility for Bucky's death" served as Cap's version of the Marvel-mandated "heroic flaw" (he was actually pretty over-the-top hysterical about it... Venture Bros. fans who recall Captain Sunshine's recent appearance? Yeah, pretty much that) before "WWII hero doesn't fit in 60s America" became more compelling; but in the long term it wound up as an important benchmark in Marvel continuity: "Bucky's death soured everyone on the concept" became the default "why we don't have kid sidekicks" explanation, and OBSD - "Only Bucky Stays Dead" - was a mantra on the ressurectability of dead heroes for decades.
Anyway, the movie question will be "what will he DO?," since the character has undergone a lot of re-interpretation over the years. Regular-Marvel Bucky has, historically, been played as a wartime Robin - all gee-whiz and let's-go. In the "Ultimate" universe - which has inspired a lot of the Marvel movieverse - Bucky is just a regular army grunt Cap hung out with during the war (he doesn't die and Cap meets him again, as an old man now long-married to Cap's onetime ladyfriend, after being thawed out, which would probably be a KILLER scene in a movie.)
The HV story, however, may unwittingly give away a pretty good idea of where they might be going by noting that Sebastian has signed for "five or six" movies as the character. See, since it's unlikely that Sebastian would sign to play himself in old-man makeup in six movies, it seems like Bucky might be running around "young" in future Cap and/or Avengers movies. How might that work?
Well, a few years back when Ed Brubaker began what's become an acclaimed run on the Cap books, he started off with a MAJOR reworking of Bucky's official backstory: Now, the "gee-whiz" kid thing was an act, and his "real" function was as Cap's one-man "wetwork" team; i.e. he did all the throat-cutting and guard-neutralizing that the Army didn't want to tarnish Cap's symbolic stature with. In addition, well... he threw out the "Only Bucky" death rule once and for all: As it now turns out, Bucky was revived in the future, too: As a brainwashed, semi-cyborg Soviet assassin called "Winter Soldier;" who Cap discovered, fought and rehabbed into a good guy... good enough that he briefly took up the Captain America mantle when Cap died after "Civil War." In other words, add at least ONE prospective story-point/bad-guy to the Cap/Avengers sequel rumors.