In Knives Out, Marta vomits whenever she tells a lie. It's an interesting quirk that makes it difficult for her to execute Harlan's plan while evading B. Blanc's suspicions. When she is in the study with Harlan, she claims that she cannot play Go because she had too much champagne. It seems the movie goes out of its way to show us that this was a lie: we see in a flashback that she had refused the champagne. This suggests that her vomiting quirk is fake, she does it whenever she wants. But this causes a contradiction: why would she vomit in the soda cup when no one was even there to see it?
Because Marta's lie-induced vomiting is likely a guilt-based physical reaction to emotional stimuli. She doesn't get sick because she's lying, she gets sick because she's a good person, and lying makes her feel so guilty she vomits. Telling Harlan she's too drunk to play Go wasn't intended to fool or deceive him, it was a weak excuse that she knew he wasn't going to fall for to get out of sitting down for a game she wasn't in the mood to play. There was nothing to feel guilty about, because she wasn't trying to hide anything. Just making idle protests to get out of playing the game, which was never going to work, and which she knew wasn't going to work.
Marta vomits when she directly lies. She gets only nauseous when she conceals the truth. While remaining in the lie, she gets sicker and sicker. At the end of the movie, she was able to hold off vomiting for an extended period of time. But, while facing the person to whom she was lying, she could not hold off vomiting indefinitely.
The simplest answer is that it likely isn't a lie at all. First, in the screenplay, it seems there was originally an intention of showing this explicitly. Marta first declines a glass of champagne but then after the awful conversation with the family about immigration where she's put on the spot, the screenplay had this:
Marta takes the opportunity to slip into the hallway, alone. She breathes hard. Takes a champagne flute from the tray. Drinks it in one gulp.
Now, I know many times the final cut of a film leaves out a detail from the screenplay for a reason, so we can't assume necessarily that Marta was still intended to drink a glass.
Nonetheless, the dialogue suggests that Harlan may have actually witnessed this "one glass" of champagne:
Marta: I had a glass of champagne. Harlan, listen to me. We are... It's late. I had...
Harlan: You've only had one glass.
Marta: Exactly. A glass of champagne.
The way Harlan says this, he states it matter-of-factly (not in the tone of a question), seemingly confirming that he knows it was only one glass. Such as, perhaps, a glass that she insisted she drink as part of a birthday toast perhaps or something.
If she were actually trying to convince Harlan she was too drunk to play, explicitly saying it was only one glass isn't going to help her case. So, I don't think there was any intent at deception there.
Given the tight editing of the film in general, it seems unlikely that the apparent on-screen contradiction was a total oversight. A few things could be going on:
- This was a possible red herring, initially casting a bit of doubt on Marta's account. But since we never get later confirmation of her ability to lie without vomiting in the film, this turns out to be a red herring.
- Perhaps for some other reason they decided to eliminate the scene of her drinking the champagne off to the side (perhaps to eliminate the implication that she was being irresponsible when she had to give medication later), but the shoot was already completed and it would be too difficult to stage another scene with all the actors showing her toasting or something instead and basically being goaded into it.
- Perhaps it really is a continuity error, though I think that's unlikely as I said above.
I know that this one detail has spawned many internet conspiracy theories about Marta. But the question is to what end would she even lie here? If she really wanted to get out of the game of Go, she could have said she had a few glasses. (Though again, that might look bad in terms of making her look unprofessional and possibly negligent for drinking before giving medication.)
So why even bother to show this supposed "lie"? Why have Harlan be the one to seemingly confirm it was only one glass (as if perhaps he knew and had witnessed it, and knew she took her job seriously enough not to drink more)?
And if she was supposed to be able to suppress her need to vomit after lying, why not show any other scenes of it in the film?
It's a puzzle, but given the screenplay and dialogue, I just assume that she did have only one glass (because she's a responsible nurse, which is what the other declining scene is there to show), and we for some reason didn't see that one glass on-screen.