To overact is to play a part with too much emotion or too much movement — big tears, a huge voice, arms that never stop — so the acting stops feeling true and starts feeling like a cartoon. People say it when they do not like the performance; it is a critic’s frown, not a neutral description. You hear it about plays and films, and sometimes, a little loosely, about real people who put on a huge show of shock or sadness. Drama teachers use it a lot. The idea on the other side is to underact, to do too little, though that word is rarer.