CGI

I was watching some behind-the-scenes stuff for this movie, and was shocked at just how much CGI was actually used in the movie. They showed shots of, say, Eliza walking over to the canal, or along the streets of 1960s Baltimore, and parts of the final picture drop into place, and it's like, "Wow! They had to completely rebuild everything on the computer!"

Or with the Amphibious Man, most of what you see of him is Doug Jones in a suit, but they had to add things like his eyes blinking, or swimming gracefully in the distance, using CGI.

Del Toro does say this was one of his "smaller" films to date, but a lot of work still went into making it, if the post-production dept. has anything to say about it.