Acting for Two: Richard Divizio as Kano and Kabal, Sal Divita as Nightwolf and the robot ninjas, and John Turk as Shang Tsung and Sub-Zero. John Turk and Becky Gable both played all the masked ninjas in Ultimate as well.
Ascended Fanon: Ermac was originally a bit of misunderstood text on a debug screen in MK. Fans believed he was going to be a playable character. Midway took the playable bit and canonized it. Likewise, his palette (red) was merely a graphical error. The costume used to record the animations for the masked fighters in the original game was red and if the game couldn't load the right color, it would default to this. When he was made an official character, he was given that same red hue.
Romance on the Set: With Lia Montelongo (Sindel) and Brian Glynn (Shao Kahn), who dated until 2001. It may influenced in the fake marriage between the two characters.
The Other Darrin: Only half of the cast from the previous two games returned, namely Richard Divizio (Kano and Baraka), John Parrish (Jax), Tony Marquez (Kung Lao) and Brian Glynn (Shao Kahn), which led to the other returning characters being played by new actors. Ho Sung Pak (Liu Kang) and Elizabeth Malecki (Sonya Blade) were replaced by Eddie Wong and Kerri Hoskins respectively, while John Turk replaced both, Dan Pesina (as Sub-Zero) and Philip Ahn (as Shang Tsung).
John Turk eventually got to play all the other masked ninjas in Ultimate (replacing Pesina again) alongside Becky Gable, who replaced Katalin Zamiar as Kitana and the other female ninjas.
Chris Alexander was brought in to replace Dan Pesina as Johnny Cage in Trilogy, who was one of the few characters needed to fill the entire roster from the first three entries. Averted by Carlos Pesina, whose character Raiden was left out in MK3 and Ultimate, but brought back in Trilogy with the same sprite set used in MK2 (except for the all-new running animation); same as Baraka, who had been played by Richard Divizio and, therefore, came back with his MKII sprites intact, plus a run animation and a new move.
This is also the reason why only four characters (the aforementioned Kano, Jax, Kung Lao and Raiden) have "past" versions of themselves available in the Playstation and Saturn versions.
Updated Re-release: Ultimate Mortal Kombat 3 is Midway's apology for not including Scorpion in the original Mortal Kombat 3. As it's the only version of the game to have been included in the Arcade Kollection re-release, it's widely considered the definitive version of Mortal Kombat 3.