That particular information has yet to be revealed, but recently during The Hashtag Show (via IGN), it was rumored that Legendary is considering four different actors to potentially voice the character. What’s particularly interesting is that this list contains actors that are probably more known for their roles in action films.
According to the report, Hugh Jackman, Ryan Reynolds, Mark Wahlberg, and Dwayne Johnson are apparently in consideration and are the studio’s top choices to take on the role, although it should be noted that none of the actors have yet to take any meetings regarding the role, so while they might be a top pick, whether or not they’ll actually take up the role remains to be seen.
Production of Detective Pikachu is expected to begin in London in January next year, which means that a release will probably be in 2019 at the earliest.