Somewhere in the foggier bits of my pre-Sundance planning brain, I remembered reading that there was an Olsen sister -- not one of the twins but a mysterious third sister named Elizabeth -- who had a movie I should check out while I was there called 'Martha Marcy May Marlene.' Fast forward to Thursday night at midnight, the end of my first full day at Sundance, and I was sitting two rows behind Harvey Weinstein at the first screening of 'Silent House.' (You can read my review of 'Silent House' here.)

Although I found 'Silent House' ultimately unsatisfying, I was immediately impressed by the lead actress, who was in nearly every shot from the very first frame. The way she hunched down into herself like an insecure child, her sleepy eyes that would light up at a joke or grow huge with terror as unknown horrors shambled around her, the way she imbued even some of the silliest lines in the movie with a sort of earnestness -- it was damn impressive. That was Elizabeth Olsen.

The name didn't connect until afterwards when we had a Cinematical pow-wow and discussed 'Silent House.' Then I realized I would be seeing 'Martha Marcy May Marlene' the next day -- an anticipated drama about a young woman who escapes from a cult and a 180-degree departure from the late-night spook show I'd just seen -- and got the definite feeling Olsen would be shaking up Sundance.

'Martha Marcy May Marlene' is superb, and Olsen's performance as the title character -- born Martha, renamed Marcy May at the Spahn Ranch-like farm she lived on with the rest of her new family, and identifying herself as Marlene on the phone at the farm when anyone called -- is the lynchpin. Olsen expertly portrays a young woman subtly broken down and brainwashed by Patrick, a Manson-type leader played by John Hawkes, who is excellent; she finds herself doing things she would have never otherwise done, and then convincing other women to do them as well.

Although she eventually finds it within herself to call her estranged sister Lucy (Sarah Paulson) to come pick her up and take her away, the damage is extensive and lasting. As she struggles to readjust to everyday life with Lucy and brother-in-law Ted (Hugh Dancy), her memories of life on the farm and their increasing violence bombard her day and night. Her boundaries have been readjusted so radically by the cult that she horrifies Lucy and Ted with her lack of social propriety; stripping nude to swim in the lake outside of their fancy Connecticut rental in the middle of the day is just the tip of the iceberg.

Olsen's dexterity in switching between the post-cult Martha, who is angry, exhausted, and increasingly paranoid, and Marcy May, who is obedient, open-faced, warm, and desperate for Patrick's approval at any cost, is extraordinary. Even as Martha, Olsen's warmth and charm comes to the surface when she says something totally inappropriate but true and laughs loudly like the rebellious teen she never grew out of being. You just cannot look away.

Unlike other Sundance breakouts, however, the way we're writing and talking about Elizabeth Olsen is complicated and a little icky. It's both unfair and impossible to avoid mentioning Olsen's siblings are Mary-Kate and Ashley, if only to give context. There's a sort of gross and guilty undercurrent of surprise that not only do they have another sister, but that she's an actress, and a very talented one to boot. In fact, that was pretty much my reaction, and it's not something I'm particularly proud of, though I'm no less delighted to have another whipsmart starlet on the horizon.