Robert Buckley as Captain America
Out of all the actors who are in the running to play Cap, Buckley above all is the best play Cap plus he's just to old and tall to play Bucky
Chace Crawford as Bucky Barnes
Though he's in the running to play Cap, I think Crawford is much better suited for the role of Bucky, and with the current Marvel trend of casting an actor in a role other than the one they intended, he may very well be casted already as Bucky.
Ian McDiarmid as Dr.Josef Reinstein
Dr. Reinstein is the man who performed the procedure that created Captain America. I think McDiarmid is a good fit for the role. Apologies for lack of character picture.
Sam Elliot as General Chester Phillips
Chester Phillips is the U.S. Army General who offered the super-soldier experiment to Steve Rogers. Again, sorry for lack of character picture.
Keri Russell/Shirley Jones as Margaret "Peggy" Carter
As she is Steve Rogers wartime love interest, its more than certain that she'll be featured in the film. I casted her as both old and young just in case she appears as both old and young or just young.
Ryan McPartlin as U.S. Agent
Its likely that U.S. Agent will be in the film along with Cap as a part of the Invaders. And even though he's a higly popular choice for cap,I casted Ryan Mcpartlin because he's a good actor and he kinda of looks like U.S. Agent
Sam Worthington as Union Jack
Union Jack, is also a prominent part of the Invaders and will also probably appear in the film. Worthington can't play Cap, but he could surely pull off this role.
and finally....
Mads Mikkelson as Red Skull
There isn't much explantion for this one. Mads for Red Skull
and Ralph Fiennes as Baron Zemo
Again, not much explaination needed here
And I must not forget....
Sam Jackson as Nick fury, and Sienna Miller as Sharon Carter aka Agent 13
Nick Fury and Agent 13 are probably are going to appear at the beginning and end of the film looking for a frozen Steve Rogers, so its proper to cast them.