Scarlett Johansson (The Prestige) The Prestige

Olivia Wenscombe is traded between magicians like a prop and the film knows it

Scarlett Johansson plays Olivia Wenscombe, Angier's stage assistant who is sent to spy on Borden and instead falls in love with him. Olivia occupies a structural role the film is honest about: she is a pawn. Angier sends her to infiltrate Borden's operation; Borden hires her knowing she was sent as a spy; each magician uses her as an instrument of espionage, and neither values her beyond her utility. She eventually vanishes from the story when neither man has further use for her. (wikipedia)

Nolan was "very keen" for Johansson to play the role. When he met with her, "she just loved the character." (backstage)

Olivia serves the story's argument about misdirection

Cutter introduces Olivia with a line that doubles as a structural thesis: "A pretty assistant is the most effective form of misdirection." The statement applies literally within the magic shows and metaphorically within the plot -- Olivia's movement between camps distracts both magicians from seeing the larger picture. Her love for Borden is genuine, but it is also structurally functional: it is the mechanism by which Borden's encrypted diary reaches Angier.

The diary delivery is the pivot. Olivia brings Angier Borden's notebook -- the document that contains the keyword "TESLA" -- as her last act of loyalty to her original employer. She confesses: "I have fallen in love with him." Angier responds: "I don't care about my wife, I care about his secret." The exchange strips away any pretense that the relationships in the film are valued for their own sake.

Johansson had limited material and the role remains a point of critical discussion

Critics noted that Olivia is underwritten relative to the male leads. Christopher Priest praised Johansson's presence but noted the limitation: "Scarlett Johanson was not given enough to do, but looked lovely and made the best of a difficult part." Brian Eggert of Deep Focus Review identified the underdeveloped female characters as "secondary concerns for Nolan." (christopher-priest.co.uk, deepfocusreview)

The limitation is arguably part of the film's argument rather than an oversight. Both magicians treat the women in their lives as instruments -- Angier uses Olivia as a spy; Borden uses her as a substitute for the twin who loves Sarah. The film does not endorse this treatment, but it does not transcend it either.