One of the strangest things about the new movie Pokémon Detective Pikachu is that Pikachu has the voice of Ryan Reynolds. Another strange thing about Detective Pikachu is that Pikachu can talk at all. If you're familiar with Pokémon games, movies, or the TV show, you know that the way Pikachus normally talk is just by saying "pika pika" over and over again. Yet, in this new movie, the main Pikachu can carry out full conversations. What gives?
In one of the trailers for the movie the lead human character, Tim (Justice Smith) expresses disbelief that the Pikachu he stumbles on can talk. Pikachu says, "Whoa, did you just understand me? Oh my god, you can understand me! I've been so lonely." Clearly, in this world, just like in every other Pokémon world, humans don't normally speak the same language as Pokémon. In a clip that comes right after, Tim asks some random people on the street if they can understand Pikachu and all they hear is the "pika pika" Pokémon fans are used to.
Pokémon Detective Pikachu is based on the Nintendo 3DS game of the same name in which the titular Pikachu also talks. So, a simple answer for why Pikachu talks in the movie is because he talks in the game.
The game was meant to really switch things up as far as what people knew of the character. Hiroyuki Jinnai, the producer of the "Detective Pikachu" game told The Verge, "We really started with the concept of making Pikachu talk. We really worked hard to come up with a justification and setting to make that work." The site explains that the "goal was to surprise people and alter the perception of the franchise’s most well-known face." Jinnai added, "It took a lot of convincing to let us break the rules." In the game, Pikachu is meant to seem like a old school 1930s or '40s detective and has a husky voice.
So, to continue with this same story — Pikachu being a detective who solves a mystery with a guy named Tim — it makes sense that he talks like in the game. The movie's adapted directly from this particular game, not the Pokémon series or one of the original Pokémon games.
That said, there is a reason given within the movie for why Pikachu talks and it's a lot to take in. If you're avoiding Detective Pikachu spoilers, turn back now.
In the film, the story is based around the fact that Tim's dad, Harry, also a detective, has either died or gone missing. The Pikachu Tim teams up with was his dad's partner. Pikachu doesn't remember what happened when he was last with Harry and he doesn't know why Tim can understand what he's saying.
We find out that the last case Harry and Pikachu were working on had to do with genetically engineering Pokémon. We also find out that Howard Clifford (Bill Nighy) — who set up Ryme City, where the film takes place, as a city where Pokemon and humans could coexist — wants to transfer his consciousness into a Pokemon before he dies. Do you see where this is going? Drum roll, please: Pikachu is Harry. The movie explains it in more detail, but his consciousness was transferred into the Pikachu and is later separated back out. As for why Tim can understand the Pikachu when he talks while no one else can... um, their familial bond?
Interestingly, the game Dectective Pikachu does not fully answer the question of what happened to Harry at the end, as noted by players on Reddit. This thread explains that the game hints that Harry is in Pikachu, but doesn't straight out say it. A couple Reddit users even guessed that the mystery would be answered in the movie.
So, there you have it. From the practical to the "Holy cow, a human is in a Pokemon?!" the two-part answer for why Pikachu can talk in the new live-action/CGI movie.