In Voices In My Head, players step into the courtroom and the mind of a man on trial for committing a bank robbery. One player takes on the role of the prosecutor, trying to convict the defendant, Guy, and send him directly to prison; all other players take on aspects of Guy's personality, such as Selfishness or Honesty, and attempt to influence the trial outcome. To win the game, each player must achieve their specific hidden goal.

Each round, the prosecutor plays a trial card to present their evidence, call witnesses, or grill the suspect on the stand. Players then try to control Guy's actions by deploying control markers to a three-dimensional game board that represents his brain. These actions will sway the jury in different directions and ultimately determine whether Guy goes free or is dealt a guilty verdict and sent to prison.