Speaking to Digital Spy, Alexander said that the Marvel sequel will see Sif "very much in love" with her childhood friend Thor (Chris Hemsworth).
"You definitely see a more vulnerable side to Sif and you see her sacrifice a bit for Thor because she is very much in love with him," Alexander said. "She cares about him, they grew up together."
"I think the movie is going to have a very much darker feel than Kenneth Branagh's Thor. We get to explore different places. It's been a lot of fun."
Thor: The Dark World will open in cinemas on November 8, 2013. Alexander can be seen in The Last Stand now.
Photo gallery - 25 biggest blockbusters for 2013: