Straight from the mouth of Marvel itself (or at least their website):
"Jaimie Alexander will make a special guest appearance as Sif in an upcoming episode of “Marvel’s Agents of S.H.I.E.L.D.,” reprising her role from Marvel’s “Thor” and “Thor: The Dark World”!"
Naturally, nothing has been revealed about why she’s come to Midgard, aside from it being a special mission. But this is what fans have been wanting since the start of the show–a tie-in to the movies. Nothing big, nothing earth-shattering, but something that still brings them together. Yes, we got Nick Fury at the end of an episode for about thirty seconds.
The Thor movies have a special place in my heart. I went into the first one not expecting much (not knowing much about the Thor plot line) and was thrilled with it; Thor: The Dark World I saw twice in the span of four days, and it would have been three times in the span of six had my sister-in-law not interfered.
Lady Sif is a very underrated character in those movies. It will be fun to see how they use her in the television show–and how much she’s in.
Besides, who isn’t excited for superhero guest stars?!