Photo of person to fine, detailed line drawing

[rich2005]

As with most things graphical it all depends on the original image, never any hard-n-fast rules.

What you can try is starting off with the gimp_gmic_qt plugin - http://www.gmic.eu - the example uses the Artistic > Illustration Look (there are others) to get a smoothed effect. 

Then the Gimp Destaturate -> Color to Gray using a small radius, about 24 (default is 300)