It does appear, however, that Farrell will be performing the role with the use of prosthetics. This was confirmed by Mrs. Doubtfire actor Rob McClure, who actually saw the prosthetics being constructed. Aside from his physical appearance, Farrell has teased some very “tasty” scenes for Penguin, and he’s also had nothing but praise for Matt Reeves’ script.
Article source: https://www.cinemablend.com/news/2553082/why-batman-returns-danny-devito-thinks-colin-farrell-will-be-great-as-the-penguin-in-the-batman