According to the fine folks over at TVLine, this is a casting breakdown of the characters that will make up the Joss Whedon-helmed S.H.I.E.L.D. TV show being spun off from the mega-successful Avengers feature film.
Being the rabid Whedonite that I am, I immediately started picturing Whedonverse actors in the roles.

Skye:   This late-20s woman sounds like a dream: fun, smart, caring and confident – with an ability to get the upper hand by using her wit and charm.
Cast: Eliza Dushku (Faith on Buffy the Vampire Slayer; Echo on Dollhouse)

Agent Grant Ward:  Quite the physical specimen and “cool under fire,” he sometimes botches interpersonal relations. He’s a quiet one with a bit of a temper, but he’s the kind of guy that grows on you.
Cast: Adam Baldwin (Jane on Firefly; Marcus on Angel)

Agent Althea Rice:  Also known as “The Calvary,” this hard-core soldier has crazy skills when it comes to weapons and being a pilot. But her experiences have left her very quiet and a little damaged.
Cast: This part seems tailor made for Gina Torres (Zoë on Firefly; Jasmine on Angel) but since she already has a gig on the really awesome USA series Suits, I went with Dichen Lachman (Sierra on Dollhouse)

Agent Leo Fitz and Agent Jemma Simmons:  These two came through training together and still choose to spend most of their time in each other’s company. Their sibling-like relationship is reinforced by their shared nerd tendencies – she deals with biology and chemistry, he’s a whiz at the technical side of weaponry.
Cast: Fran Kranz (Topher on Dollhouse) and Amber Benson (Tara on Buffy the Vampire Slayer)