Let's start with the basic figure face.
The problem with fine detail on figures is that you often need to create minute elements that are smaller than a single drop of paint on the end of a brush tip. There are ways around this.
The first thing I do is to apply a white dot over the entire area of the eye. It doesn't matter if this dot is bigger than the final eye (in fact it's probably good if this dot is too big). If i'm doing something more exotic (like a cat's eye), I might colour this sclera in yellow, red, or some other shade.
Next we determine the colour of the iris. This can be done with a simple stripe of colour across the white sclera, the placement of these lines may determine the direction the figure is looking once the eyes are finished.
Drawing a vertical slit takes practice, but it's not too hard after a few attempts. I don't bother with the black pupils, because my hand just isn't steady enough, and because these are figures for playing with, not for close examination.
Next, I take a darkened shade of the skin tone, perhaps mixing in some black or an eyeshade colour if I'm painting a figure with make-up on. With this colour I paint over the edges of the white sclera to give the eye it's intended shape.
This is why the original white colouring doesn't matter if it spreads too far. If you wait until the first layer of paint has completely dried when you do this, you can leave a very thin slit for the eye. For some figures, I'll look at some art books to get inspiration for eye expressions (angry eyes, worried eyes, sad eyes, surprised eyes, etc.) these all vary with the eye outlines.
Next I blend a colour between the eye outline and the skin shade, to graduate the eye outline toward the figure's skin-tone. An outer line, overlapping the existing eye outline makes the outline finer.
In figures with dramatic make-up or stronger shadows, I might keep the darker outline larger above the eyes, perhaps stretching it as far as the eyebrow ridge. Books on make-up and fashion spreads from newspapers/magazines/websites can give some great ideas for facial colourings in this regard.
The final thing I do before this paint dries is blend it into the existing skin-tone.
I've always thought that the eyes can make or break a figure. I might not get them right every time, but a bit of effort gives some character and personality.