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. Liam McIntyre came onto the series severely underweight from another role, and had to bulk up.
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.
The Other Darrin: As of the second season, Liam McIntyre will be replacing Andy Whitfield as Spartacus, while Cynthia Addai-Robinson will be replacing Lesley-Ann Brandt as Naevia due to Brandt's preoccupation with CSI: NY.
Playing Against Type: To most American tropers, 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.
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.
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.