Warning: spoilers ahead for Guillermo del Toro's The Shape of Water!
Constructing a proper character for a film requires input from several different sources. A lot of it comes from what's written into a script, but it also takes what an actor puts into the performance. In the case of Sally Hawkins' portrayal of Elisa in Guillermo del Toro's The Shape of Water, it turns out that the actress had far more control over certain character moments than we could've anticipated. In particular, del Toro recently opened up and admitted that two of these character moments came from a similar story that Hawkins had been working on before joining the film. Del Toro explained:
It was so beautiful that we were on the same wavelength. I asked her if I could use this idea that she had scars on her neck that turned out to be gills. She allowed me to use another detail she had, which was that the character used a lot of salt to make the water in her bathtub habitable.
That's a reasonably bizarre coincidence, but it arguably ended up working in The Shape of Water's favor in the end. Sally Hawkins had already thought of a character who was unaware that she was a mermaid, and several of those beats ultimately helped flesh out Elisa's arc and the way in which she treated her love affair with the creature. Between the gills and the salt, she tapped into Elisa in a way that wasn't initially on the page.
Those character moments go a long way towards crafting the story that critics seem to have fallen in love with, and that shows in The Shape of Water's critical accolades. The R-rated fairy tale has gone on to earn immense praise since its release, and the awards consideration has already begun in the form of seven Golden Globe nominations, including Sally Hawkins' nomination for Best Actress in a Drama.
That said, one thing that Guillermo del Toro didn't explicitly explain in his conversation with Deadline is how Elisa's gills factor into the story. The film makes it clear that Elisa was found near water as a baby, which lends credence to the possible theory that she is actually from the ocean, just like the creature. However, another viable reading of the film's ending is that the creature turns her scars into gills (rather than restoring gills that may have previously been there) and grants her a new home in the water. Both of these possibilities seem to potentially make sense based on what the film gives us in its final moments, but if you are still curious, then maybe it could not hurt to find out what Sally Hawkins' chose for her character in her story.