'S.H.I.E.L.D.,' assemble! While we still don't know how exactly Clark Gregg will return to reprise the role of Agent Phil Coulson in ABC and Joss Whedon's upcoming 'S.H.I.E.L.D.' TV series, we've at least got the first assuredly living casting. Former 'ER' doc Ming-Na will add a few new letters to her resume as one of the new agents to be featured in the 'S.H.I.E.L.D.' TV series, but what role will she play? And what's changed about the original casting?