Spoilers for ep. 17

Here is an interview with the actress who plays Emma, Jennifer Morrison, on her upcoming interactions with the Mad Hatter/Jefferson. She revealed that his character causes Emma to begin to suspect something dark is going on in SB :

The March 25 episode will see Emma kidnapped by the Storybrooke version of the Mad Hatter. What can you reveal about that episode and what he wants from Emma?

“It was probably the most fun I’ve had so far shooting episodes. It’s one of my favorite scripts. I had such an amazing time. Sebastian Stan, who plays the Mad Hatter, he’s such an extraordinary actor. It was so great coming to work with him every day. He did such cool stuff with his character. I mean, there was not a second I didn’t believe he really was this guy. So, from that perspective it was just such a good time.

It does start to plant some interesting seeds in Emma, because it’s another person saying to her some of the things that Henry has been saying. She can still justify it away in the sense that, “Well, maybe he read the book. Maybe there’s another copy of the book. Maybe he heard about the book.” But it is really the first time that there are little chips in Emma’s armor in terms of her thinking; not necessarily that there really is a curse and that everybody really is a fairytale, but I think it’s really that turning point for her to realize that there is something very dark and possibly dangerous going on that she needs to try to figure out.” http://www.huffingtonpost.com/2012/03/12/once-upon-a-time_n_1338893.html

Also, in another article we find out more about episode 17:

Sebastian Stan guest stars as the Mad Hatter and the Who’s Roger Daltrey voices a talking caterpillar in the March 25 episode of Once Upon A Time!

Also in the episode, Emma is kidnapped by a man whose sanity teeters on the edge of madness — how much do you want to bet that’s Sebastian Stan’s Storybrooke alter ego — and the Evil Queen persuades a man of magic to steal something from the Queen of Hearts.

In more spoiler news, Mary Margaret is missing by this episode and the Evil Queen’s interaction with the Queen of Hearts brings her to Wonderland! http://www.ksitetv.com/once-upon-a-time/once-upon-a-time-episode-17-hat-trick-images/12569