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.