The flashbacks show her going to the island as a sweet, enthusiastic person. As time goes on we see how she was manipulated into staying on the island. Ben talks her out of leaving at first before he tells her she is staying. We see how Mikhail used The Flame station to monitor what was going on in the outside world. A camera was used to film Juliet's sister Rachel on the very day that the plane crashed.
Back on the island everyone seems to have it in for Juliet from the moment Jack leads her back to camp. He seems to trust her completely, but the other 815ers don't seem very keen, Sawyer and Sayid in particular. In the midst of Jack's return to the camp, Claire becomes very ill (did you notice how no one was ill at all whilst Jack was away?!) Juliet knows why. She reveals that all the children and their mothers who have conceived on the island have died. Now we know why the Others want the children! She was brought to the island to try to prevent the deaths. We see how Ethan took Claire so that Juliet could experiment with her. Juliet managed to keep both baby and mother alive. When Claire starts to feel ill Juliet knows how to stop it, by giving her some medicine.
Another flashback then shows how Juliet is double crossing everyone. She has been sent to the camp for a reason (of which we don't yet know). She activates an implant in Claire which causes an illness which she can cure to look like a good guy. Her plan is to infilrate the camp and become accepted.
This all begs the question - who is she lying to - Jack or Ben? She is double crossing someone, and at the moment it seems to be Jack. Dammit.
Next week it's a Desmond flashback!