Why Orphan Black's Tatiana Maslany deserves an Emmy nomination: Each clone has her own mannerisms, her own way of speaking, her own posture. Even if you came into an episode halfway through, you could likely tell which character Maslany was playing at a glance. It's easy to forget that Alison and Sarah, who have many scenes together, are played by the same actress. I've often found myself grasping for the name of the actress who plays Cosima, only to hit myself on the head and remember that Maslany plays her as well.
Maslany's ability to make each character so distinct is never more clear than when the clones pretend to be one another. Alison pretending to be Sarah is still recognizably Alison and vice versa. If you're not averse to spoilers, this is Sarah: