There’s an open casting for extras going on right now in London for filming later in the year. Applicants need a UK National Insurance number and should send a resume and photo (not over 2 meg) to firstname.lastname@example.org. You can also post a resume and photo to Guys and Dolls Casting, Trafalgar House, Grenville Place, Mill Hill. London NW7 3SA.
Marking your interest in Captain America would probably be a good idea.