[1] : WordNet (r) 2.0

fill in
     v 1: supply with information on a specific topic; "he filled me
          in on the latest developments"
     2: represent the effect of shade or shadow on syn: shade
     3: be a substitute; "the young teacher had to substitute for
        the sick colleague"; "the skim milk substitutes for
        cream--we are on a strict diet" syn: substitute, sub,
         stand in
     4: write all the required information onto a form; "fill out
        this questionnaire, please"; "make out a form" syn: complete,
         fill out, make out
see also:
shade substitute sub stand in complete fill out 
make out 

[2] : Moby Thesaurus II by Grady Ward, 1.0

103 moby thesaurus words for "fill in":
   accomplish, acquaint, act for, admit, advise, alternate, apprise,
   assimilate, bring to completion, bring to fruition,
   change places with, clue, complete, comprehend, comprise, contain,
   count in, cover, crowd out, cut out, displace, do a hitch,
   do a stint, do a tour, do time, double for, eke out, embody,
   embrace, encircle, enclose, encompass, enlist, envisage, fill,
   fill in for, fill out, fill up, fulfill, ghost, ghostwrite,
   have a go, have tenure, hold, hold office, include, incorporate,
   insert, insinuate, intercalate, interject, interpolate, interpose,
   keep a watch, locum tenens, make good, make up, mature, notify,
   number among, occupy, piece out, pinch, pinch-hit, post, re-up,
   receive, reckon among, reckon in, reckon with, reenlist, refill,
   relieve, replace, replenish, represent, round out, serve time,
   shallow, shoal, sign up, silt up, spell, spell off, stand in for,
   subrogate, substitute for, succeed, supersede, supplant, surrogate,
   swap places with, take in, take into account,
   take into consideration, take turns, take up, tell, throw in,
   time off, top off, understudy for, warn

Fill >> F Words
Fill, definition of term: Fill
fill_pag1.html

Fill Up >> F Words
Fill Up, definition of term: Fill Up
fill+up_pag1.html

Fill In >> F Words
Fill In, definition of term: Fill In
fill+in_pag1.html

Fill Out >> F Words
Fill Out, definition of term: Fill Out
fill+out_pag1.html