The Collaborative International Dictionary of English v.0.48:

Overact \O`ver*act"\, v. t.
   1. To act or perform to excess; to exaggerate in acting; as,
      he overacted his part.
      [1913 Webster]

   2. To act upon, or influence, unduly. [Obs.]
      [1913 Webster]

            The hope of inheritance overacts them. --Milton.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Overact \O`ver*act"\, v. i.
   To act more than is necessary; to go to excess in action.
   --B. Jonson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overact
    v 1: exaggerate one's acting [syn: overact, ham it up,
         ham, overplay] [ant: underact, underplay]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "overact":
   act a part, affect, assume, carry too far, counterfeit, declaim,
   do a bit, dramatize, exaggerate, exhaust, fake, feign,
   go overboard, go to extremes, go too far, grimace, ham, ham it up,
   histrionize, know no bounds, make out like, mug, out-herod Herod,
   overcarry, overdevelop, overdo, overdramatize, overdrive,
   overelaborate, overemphasize, overexercise, overexert, overexpend,
   overindulge, overlabor, overplay, overreact, overspend, overstrain,
   overstress, overstudy, overtask, overtax, overtrain, overuse,
   overwork, pass all bounds, play, play a part, play a scene,
   playact, pretend, protest too much, put on, put on airs, rant,
   roar, sham, simulate, spout, throw away, tug the heartstrings,
   underact, wear