So what do you say about someone who’s perfect? No, this is not just fannish hyperbole… just that the more symmetrical (and hence attractive) the features, the harder it can sometimes be to figure out what makes someone look different than someone else.
Anna Torv is in many ways straight out of your “How to Draw the Human Head” book. Her eyes are spaced exactly one eye-length apart, which is what textbooks will start you with (and to which many people’s actual features often do not conform). The most remarkable thing about her nose is that there is nothing remarkable about it! Surprisingly, trying to draw a perfect nose can sometimes result in a drawing that just does not seem right. Solution: don’t draw her nose! More specifically, draw as little of her nose as possible. The reason for this is that the mind’s eye tends to fill in any symmetry that we expect to see. Sometimes, suggesting a feature is much better than actually defining it with many lines. Pay attention to the proper length of her nose, and you should be fine.
There is also nothing remarkable about her hairline, which is also at the textbook location (and shares the general principle of “hair is darker closer to the face”), nor the generally oval shape of her face. (As for her hair, she can have many styles, and I would just direct you to any general tutorials on how to draw hair.)
So what makes her Anna Torv? In my opinion, the eyes.
Anna is partly of northern European descent (Estonian family background) and her eye shape definitely reflects that. Northern Europeans tend to have narrow eye openings (eye shape) — as opposed to round — where the eyes themselves may be large, but the aperture is not so large. (There is a difference between eye size and aperture size which varies with each person.) When Anna Torv smiles or laughs, this is very apparent as her eyes get very “crinkly.” You may have also noticed that her looks change dramatically depending on how much eye makeup she is wearing. As Olivia, she typically wore very little eye makeup, so pay attention to the shading and line weights around her eyelashes.
One last word of warning about drawing Anna/Olivia: When looking at reference photos, be aware that she has often been the victim of genuinely terrible Photoshopping jobs by
Fox/WB publicity departments! Don’t ask me why! But photos taken in person or at events probably are better choices.