Original lead star Andy Whitfield was diagnosed with cancer immediately after the inaugural season aired. This necessitates a Prequel second season to give him recovery time, and when that ultimately didn't happen, he was recast in the final two seasons. He passed away immediately after Vengeance finished production.
SFX Lead editor Grant Konfeld passed away sometime before the final season.
Jeffrey Thomas (Titus Batiatus) and Stephen Lovatt (Tullius) in Gods of the Arena.
Pane Hema Taylor (Nasir), Peter McCauley (Lucuius Caelius) and Hannah Mangan-Lawrence (Seppia) in Vengeance. Taylor was promoted the following season.
Gwendoline Taylor (Sybil) in War of the Damned.
Breakthrough Hit: The TV series significantly raised Steven S. DeKnight's profile as a showrunner.
Cast the Expert: For the orgy scenes, a professional New Zealand company that specialises in sexual gatherings was hired. They were told to do whatever they wanted on camera, and anything that was too much for the network would be edited.
Corpsing: The infamous Bed Trick scene in the episode "Whore" took twelve hours to film, not helped by the fact that Andy Whitfield and Viva Bianca kept bursting into hysterical laughter while their characters were having sex.
Dyeing for Your Art: The actors playing the gladiators had a strict boot camp and diet to ensure their physiques. The women had a strict diet and workout too, but obviously not the extent of the gladiator one. However, there are specific examples:
Liam McIntyre came onto the series severely underweight from another role, and had to bulk up.
Manu Bennett actually grew his hair and beard twice for the series.
The naturally blond Anna Hutchinson had to dye her hair red to play Laeta.
Fake Nationality: Nobody is from any of the areas of the Roman Republic where their characters would have originated:
Welsh-Australian Andy Whitfield and Australian Liam McIntyre play a Thracian, who inhabited south-eastern Europe, beyond the borders of Ancient Greece.
In Memoriam: The first episode of Vengeance had an ending dedication to Andy Whitfield, the series' original lead. He is honored again in the Grand Finale, this time along with the show's VFX lead editor Grant Konfeld.
Method Acting: Cynthia Addai-Robinson was crying for real during Crixus' death scene, using the death of her father (who had recently passed away from cancer at the time). Manu Bennett, who didn't know how he was going to play the scene, said once she told him what she would be doing, he realized he didn't have to do anything but react to her. The look of support Crixus gives Naevia is actually from Manu to Cynthia.
Playing Against Type: To most American audiences, John Hannah. Widely known as bumbling brother Jonathan in the Mummy movies, he shows a very dark side in the fifth episode. To a lesser extent, Lucy Lawless as his conniving wife. The two actors enjoyed playing against type so much that they both came back to reprise their characters for the upcoming prequel mini-series.
Manu Bennett once played Marc Antony on an episode of Xena: Warrior Princess, with Lucy Lawless of course playing the titular heroine, who in this case was under cover pretending to be Cleopatra and having the historical affair with Marc Antony in her stead.
Dustin Clare (Gannicus), Dan Feurriegel (Agron), and Nick Tarbay (Ashur) would went on to appear in Pacific Rim: Uprising.
After the show's conclusion, Manu Bennett (Crixus), Katrina Law (Mira), Cynthia Addai-Robinson (the second Naevia), Nick Tarbay (Ashur), Liam McIntyre (the second Spartacus), Todd Lasance (Julius Caesar), and Simon Merrells (Marcus Crassus) would all appear in the Arrowverse.
Varro was supposed to die in "Mark of the Brotherhood", but Jai Courtney's friendship and chemistry with Andy Whitfield led to his death being pushed back.
"Delicate Things" was initially intended to be largely a flashback episode showing Spartacus's and Sura's relationship from their first meeting until the beginning of the show.
In "Revelations", Solonius was meant to try to flee from Spartacus and die a coward, but the writers changed their minds due to the actor's good work and impressive physique, leading to Solonius fighting back and dying a warrior's death.
During Titus Batiatus' death scene, his son Quintus Batiatus was meant to be present. With his dying breath, Titus would request Oenomaus be freed. Quintus would then decide not to honor this last request.
In "Libertus", when the rebels infiltrate the arena to bring it down, it was planned that Tullius' corpse be found at one point, OR Tullius would have somehow survived, but gone mad and be eating rats in the depths of the arena. Also, the spear that killed Cossutius was meant to have gone into his mouth and out the back of his head, as a fitting Karmic Death for a rapist.
The penultimate episode, The Dead and the Dying, was originally meant to be closer to the historical event in which Spartacus would have captured several Romans and forced them to fight each other in gladiatorial combat to honor Crixus, which would culminate in a battle between Ceasar and Tiberius. The writers decided that they couldn't have the second to last episode be nothing but the protagonists sitting down watching other people fight, so it was rewritten so that Spartacus, Gannicus, and the others fought the Romans themselves.
Mira was originally going to survive "Wrath of the Gods", and would have eventually become Gannicus' lover.
Agron was planned to die in the ludus massacre instead of Duro, but producers realised it would have more emotional impact if it was the latter. This is hilarious considering Agron is one of the few characters to survive the series.
A movie was planned, taking place between Vengeance and War of the Damned, but producers realised too much would have to be censored to get an NC-17 rating in the US.
Lucretia was supposed to have died in the Season 1 finale along with everyone else. However Lucy Lawless proved popular on set and producers loved her performance so much, she was brought back for Season 2. According to Viva Bianca, Illythia was meant to die in the ludus massacre too - but when Andy Whitfield was diagnosed with cancer and Lesley-Ann Brandt chose not to renew her contract, Steven S. DeKnight wanted to keep as many familiar faces around to help viewers with the transition.
Barca was written to be Oenomaeus originally and the brotherly bond with Crixus was intended for him. However Antonio Te Maoiha was only available for six episodes, so they rewrote the character to be Barca, and then had Doctore revealed to be Oenomaeus.
Steven S. DeKnight had stories planned to go a few more years, but the network decided to end after Season 3. They told him in advance, so he could wrap up the story.
Cossutius was going to take a spear through the mouth (which would have been even more karmic), but the special effects were too costly. So it was changed to the chest.