Walter Koenig—better known as Chekov— wrote his own treatment for the film called In Flanders Field. He even planned for most of the cast to die. We like you, Chekov, but that's a bridge too far. (Source)

Klaa, an antagonist from the previous Star Trek film, makes a brief appearance as a translator during Kirk and McCoy's trial. Dude got demoted for getting beaten so badly by Enterprise. Ouch. (Source)

Before The Undiscovered Country went into development, Paramount wanted to recast the lead actors and make a prequel set during their time at Starfleet Academy. And, somewhere, J. J. Abrams was born. (Source)

Christian Slater has a low-key cameo in the film. The dude is such a hardcore Trekkie that he didn't even cash his check for the gig—he framed it. There's a nerd after our own hearts. (Source)