I'm through 5 eps on my rewatch.
I agree with you, Tatiana does an amazing job of playing a character trying to be one of the other characters. Alison pretending to be Sarah was another great moment.
But what I've noticed, aside from the great acting, is, along with the makeup on the different clones, how well her movements and facial expressions convey that these are wildly different people.
They way she moves and looks as Helena gives an eerie impression of someone who is completely mentally unbalanced.
Cosima really comes off as a sweet girl.
Alison comes off as a paranoid soccer mom.