ham it up

from WordNet (r) 3.0 (2006)

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

from The Collaborative International Dictionary of English v.0.48

Ham \Ham\ (h[a^]m), v. i. (Theater)
   To act with exaggerated voice and gestures; to overact.
   [PJC]

   {ham it up} to act in a showy fashion or to act so as to
      attract attention; to ham. [Colloq.]
      [PJC]