Lindsay Lohan -- Stage Frightful In Play Debut

Lindsay Lohan made 'em laugh during her first performance of David Mamet's legendary play "Speed-the-Plow" in London ... problem is, it wasn't supposed to be funny.

Lohan was a disaster ... unprepared, nervous and flubbing lines left and right. According to several accounts ... people backstage were feeding Lindsay her lines. Even more obvious ... she carried a prop book with her dialogue and repeatedly consulted it.

The theater was reportedly only half full.