Trivia: The "old gangster movie" that's on the TV in Harry's apartment is the fictional movie "Angels with Filthy Souls," which was seen in the movie "Home Alone." The director wanted a detective movie for the scene, and said: "We needed something and I hadn’t figured out what I was going to put on that TV yet...Just as a laugh, we got the movie that they shot for Home Alone off of YouTube...We could never beat it!"
Trivia: Reportedly The Pokemon Company was incredibly protective of the character Ash Ketchum, and wouldn't allow any references to the character at all in the movie. It was one of the only major rules they set for the production. (Along with the idea that Pokemon always had to "look adorable.").
Trivia: There's a reference to the artificially-created Pokemon Mewtwo having escaped "twenty years ago." This is a bit of an inside joke, as the film was in production exactly twenty years after the release of the animated movie "Mewtwo Strikes Back," and that film deals with what happens after Mewtwo was created and escaped captivity.
Trivia: Ryan Reynolds has half-jokingly, half-seriously said that the studio could probably make an R-rated cut of "Detective Pikachu" if they wanted, because of all the jokes he improvised during the voice and motion-capture sessions. He described the process as "you throw a billion things up against the wall...", and said he'd sometimes come up with dozens of different variations for any given joke, which would range from child-friendly to for adults only.