WordNet (r) 3.0 (2006):

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,
       fill in]
    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,
       fill in]
    4: write all the required information onto a form; "fill out
       this questionnaire, please!"; "make out a form" [syn:
       complete, fill out, fill in, make out]

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