Source : Webster's Revised Unabridged Dictionary (1913)

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

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

            The hope of inheritance overacts them. --Milton.

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

Source : WordNet®

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