A more thorough explanation:

An accomplice witness is someone who participates in a crime with the defendant and also serves as a witness to the crime. This means that they were involved in the crime in some way, either before, during, or after it was committed. However, simply knowing about the crime and not reporting it does not make someone an accomplice witness.

Depending on the extent of their involvement, an accomplice witness may be charged with different crimes, such as being an accessory before or after the fact. The Model Penal Code states that an accomplice witness must have had the purpose of promoting or facilitating the crime and must have provided some form of assistance in its commission.

Examples of qualifying assistance include solicitation, encouragement, aiding, attempting to aid, agreeing to aid, and omission. For instance, if someone encourages another person to commit a crime or helps them plan it, they could be considered an accomplice witness.

Under common law, the state must prove that the underlying crime was committed by the principal in the first degree in order to establish accomplice liability. Qualifying assistance under common law include solicitation, encouragement, active assistance, and omission.

An accomplice witness may be convicted of criminal attempt even if the crime was neither committed nor attempted by another, so long as the purpose of their conduct is to aid another in commission of the offense and such assistance would have made them an accomplice if the offense were committed or attempted.

Overall, an accomplice witness is someone who not only witnesses a crime but also played a role in its commission. They can be charged with crimes themselves and may face serious consequences for their involvement.