So ultimately, as Ryan Reynolds explained during his visit to The Tonight Show Starring Jimmy Fallon, he was politely asked to just make the character sound like himself. Or as Reynolds joked about what they told him, “Maybe shut up a bit, and do the other thing.” The benefit of taking this vocal approach is that it’s immediately clear that Reynolds is the voice behind Pikachu, so folks who weren’t initially interested in Pokemon: Detective Pikachu, but now know Reynolds is involved, might be more inclined to see it if they’re fans of his.
Article source: https://www.cinemablend.com/news/2471277/ryan-reynolds-wanted-to-do-another-voice-for-detective-pikachu-but-got-rejected