Trivia: In many European countries, as in Spain and France, since Dennis Rodman's weirdness isn't that well known, the celebrity mentioned in the last line of the movie as being an alien was changed to Michael Jackson. (01:30:00)
Trivia: After reading the script with his wife, Barry Sonnenfeld (the director) turned to his wife and said "Tommy Lee Jones". She turned to him and said "Will Smith", and that is how they were cast. This is confirmed by Barry Sonnenfeld on the DVD commentary.
Trivia: Clint Eastwood had the chance to play K but declined.
Trivia: The film won the Academy Award for Best Make-Up. It was also the only feature film of 1997 to win an Oscar without being nominated for Best Picture.
Trivia: Vincent D'Onofrio based the voice of Edgar on a combination of John Huston from Chinatown and George C. Scott from Dr. Strangelove.
Trivia: The writer of the original comic book, Lowel Cunningham, has a cameo in the Men in Black facility.
Trivia: The role of J was offered to Chris O'Donnell but he refused.
Trivia: Bruce Campbell and John Turturro were considered for the role of Edgar the alien, but both refused it.
Answer: K and his partner both knew about aliens living on Earth, and made it their duty to make sure no one else knew about it. When the trooper saw Mikey in his real form, and Mikey spotted him, he attacked the trooper in an attempt to kill him, probably to ensure that the trooper wouldn't be alive to expose them. Since K and the other agent were already aware of Mikey, as they had dealings with him before, Mikey didn't attack them.