I guess he wanted to try something different and he totally nailed it. This movie is 1 of the best I've seen in my life. Before movie was released, Carrey must have been super worried, because this movie isnt fun, yet again we've seen The Truman Show (1998), which is 1 of Carrey's best, so I guess they knew direction?
Actually, Carrey finally got it right after a few missteps. His serious films previous to ESSM; The Truman Show, Simon Birch, Man on the Moon and The Majestic, were decent performances, but not well received by critics. ESSM brough real street red to Carrey's career. It was the first movie to receive numerous awards and best movie nominations, including two Oscars; one for best original screenplay, and best actress (Kate Blanchet).
Actually Emily, The Truman Show is critically acclaimed, as it has a 94% on Rotten Tomatoes and a 90% on Metacritic. Both Truman and Eternal Sunshine are great movies and the only 2 Jim Carey movies I like.