At suppression hearings, the defense attorney bears the burden of proof that a search was illegal or a confession was coerced. The only exception involves allegations that Miranda warnings were not given, in which case the state retains the burden of proof. Suppression hearings are best characterized as swearing matches over what really went on at the police station. Judges are reluctant to grant motions for them because they are restricted to an umpire role. The defense attorney who files them may be perceived as engaging in frivolous pretrial delay, but they have nothing to lose.
Discovery seeks to ensure fairness in the adversary process.
Ambush, concealment, and surprise are not to be tolerated by the judicial system. The rules of criminal procedure require the defense to give notice beforehand if they plan to mount an alibi defense, offer an insanity plea, or call expert witnesses , and all these things may trigger a pretrial hearing. In practice, the prosecution offers to share what evidence it wants to in order to give the defense an advance glimpse at the prosecution's case in order to encourage a plea of guilty. There are cons on both sides with the discovery rules. The exclusionary rule prohibits the prosecutor from using illegally obtained evidence during trial.