I think studios use cel paint to make those 
lines - just thinned to the point that it can be 
used in a pen. Try checking out this site:

http://www.chromacolour.com/default.html

and look into. . . I think I remember they called 
them "dip pens"?? I don't have time to look now 
since I really have to get to sleep. . . ^_^;;  
In any case, those pens can be used with thinned 
cel paint.  I'd love to see how one works/feels, 
but I don't do fancels enough to justify buying 
one. ^_^;;

Many Sharp Smiles,
--Drac