Finding good references is the key to creating a good likeness. For this I always use screen captures from movies because sometimes you can find shots where the character turns, plus the camera distortion is always the same and predictable. The only drawback is the low image quality, but in my case that's totally fine because you can't find high resolution images of Ingrid Bergman anyway! (Fig.01)
I used an old base mesh and jumped straight into ZBrush for tweaking. I kept the reference image open and tabbed between that and ZBrush when sculpting. This process is a lot like life drawing, where you alternate between looking at your work and the model!
After I was satisfied with the sculpt, I imported the file into Maya and tried to match the camera with my reference. I then made a direct comparison in Photoshop by placing the two side by side. This process can take quite a while, especially if you are repeating the process with different reference images to make sure the model is accurate from multiple angles.
The eyes are important as a tracking point when matching your camera, since their size and location are usually the same on every human. So it's generally a good place to use as a reference because you know, with some certainty, that it is one of your only constants.
Try to keep the textures as simple as possible, because otherwise they will interfere with the model (Fig 02 - 04)
In my opinion the mouth area is the most important part of nailing a likeness, so I composite my 3D render into the lower part of the face on my reference image and (hopefully) it still looks like Ingrid Bergman. (Fig.05)
The Eye ball consist of 2 parts, the inner part used a SSS material and the outer part used a PhongE material. The Attribute on SSS doesn't seem to matter that much, as long as it gets the Scattering effect it should look fine. Fig.06-08)