The possessive witness's pertains to something associated with a singular witness.
The possessive witnesses' pertains to association with more than one individual witness (witnesses).
(The...
A witness is anyone who saw an event happen and can be called upon to appear in a court of law to recall that account as seen by that witness. An expert witness is one who specializes in a specific...
A "witness" will testify as to the facts that pertain to the issues of the case itself. A "character witness" testifies as to his knowledge and opinion of the character of either the accused or a...