Will Smith recently inked his deal to come back as the alien-chasing government agent, as did returning director Barry Sonnenfeld.
Most of the action in the film will be set in 1969. Josh Brolin is playing a young Tommy Lee Jones (he played the role of Agent Kay in the first two films), this will bookend the movie.
Much of the new casting will be the villains. Their identities, look and super-abilities are being kept a secret. Clement is going to play a villain named Boris, "who is described as being charmy and creepy at the same time." (Originally he was called Yaz but that was a filler name, as the script is still being polished; the name may change again.)