Definition - What does Interested Witness mean?
An interested witness is one that would have a material interest in the case through having a bias, or likely bias, toward the defendant. If two friends were being charged with the same crime and a third friend had to give evidence against the first two, the witness would be considered as an interested witness. The separation and clarification in law of different types of witnesses is done for the sake of deciding what weight to apply to the witness, i.e. if they are inclined to show bias toward the defendant they may not be called to the stand.
Justipedia explains Interested Witness
If an interested witness is called to the stand, they are usually questioned in a particular manner so as to remind them throughout that they would be relied on to be honest and would be charged criminally if they did not. If an interested witness does not give willing testimony they can be held in contempt of court if the judge so rules.