While this has been conflated over the years into "TUPAC SHAKUR WAS THE ORIGINAL CHOICE FOR MACE WINDU!", it's not quite that dramatic, but pretty weird nonetheless - Tupac Shakur was gonna audition for George Lucas for the film that would become The Phantom Menace. However, Tupac was killed before he could audition, so we never even got an awesome audition video surfacing on Youtube where Pac has to discuss space trade tariffs.
Really though, Tupac being in Star Wars isn't that out of left field - he'd been an actor from an early age, performing in a repertory ensemble's production of A Raisin in the Sun at age 12. After moving to Baltimore, he enrolled in an arts school, studying acting, music, and dancing. In other words, 2Pac knew how to act - which would have totally disqualified him from being in The Phantom Menace (take THAT, 9 year old kid who played Anakin!).
The beauty of Star Wars is that it really is a universe - through movies, TV, comics, books, and more, virtually every aspect of the galaxy has been explored AND named. Case in point: we know the planet Obi-Wan was born on, despite it really never being an important or relevant aspect in any of his stories. And better yet, his planet is named Stewjon...after Jon Stewart.
Stewart was hosting a panel at Celebration V between himself and George Lucas, and Stewart asked Lucas if he could name Obi-Wan's home planet. Lucas, kinda joking, replied "Stewjon."
And from then on, it was canon. As in, G-canon - the highest-level of canon in Star Wars. So high was this canon that it overrid previous pieces of Star Wars information that listed Obi-Wan's home planet as Coruscant. Hell, Lucas made up a planet, and the keepers of Star Wars lore had to figure out the correct spelling and placement on the galactic map AN HOUR LATER.
The Star Wars Holiday Special is a legendarily bad piece of entertainment. How bad? George Lucas is quoted as saying that - if he could - he would destroy every single copy with sledgehammer. It was only aired once, and has never been available for purchase. And yet, despite all of this, Carrie Fisher has a copy of her own that she plays every now and then.
In exchange for recording DVD commentaries for the original Star Wars trilogy, Lucas agreed to provide her a copy of the Holiday Special. She claimed to still show it at parties, "mainly at the end of the night when I want people to leave."