The rap battle scenes took days to film, and the 300 extras were starting to get bored. Curtis Hanson started an improv freestyle rap battle among them, and the three best rappers would be filmed going head to head with Eminem. Each of the 134 volunteers got 15 seconds in front of the judiciary panel. Ultimately, the jury chose 4 rappers, who got a one-shot one-take-only scene with Eminem, who wanted to mime his responses to save his voice for the scripted scenes. Eminem couldn't resist the challenge by staying silent, especially with the crowd taunting him, and took on his opponents.
Future is based almost entirely on Eminem's best friend Proof, from hosting the battles to the story of how he got his name. Proof also plays Lil' Tic, the rapper B-Rabbit faces in the first battle. Proof manages to sneak his name as an acronym when he raps, "I'll (P)unish (R)abbit (O)r (O)bsolete (F)uture."
Eminem was quite keen that the film not be an autobiographical piece as he felt that homing in on his own personal history would limit the storytelling and characterization. By creating a fictional character, he knew that he would be unrestricted in how he performed.
When Eminem lived in Detroit, he first started performing at Shelter, a Detroit club located in the basement of St. Andrews Hall. Scenes that take place at Shelter were filmed in an empty warehouse because the actual club looked too nice.
In the last battle against Papa Doc, B-Rabbit says, "he's shook 'cause ain't such thing as halfway crooks." This is a reference to the instrumental playing in the background, from the rap duo Mobb Deep's "Shook Ones Pt. II". The chorus is "There ain't no such thing as half-way crooks, scared to death, they scared to look, they shook" The same song played in the opening scene, when Jimmy is practicing in the bathroom.
In the movie, Rabbit's mother states he got his nickname rabbit because as a child he had big ears like a bunny rabbit. In real life, Eminem's relatives called him "Mickey" in reference to "Mickey Mouse," for the same reason.
Proof, the inspiration for the character Future, was sought by producers to play that role in the film. However, Proof failed to show up for the audition and director Curtis Hanson decided that he wanted a more experienced actor to play Future. Proof was then cast in the smaller role of Lil' Tic.