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]