Reliable Witness Testimonies

After hearing the testimonies from the past few days, I find it harder to decide if Officer Wilson is guilty or not.  Many witnesses that testified had a different point of view on how they saw the incident.  A question that pops up is, what kind of witnesses should we trust to give reliable testimonies?  Some of the witnesses changed their stories multiple times while others gave a single statement.  I think if a witness is able to give a single statement without changing any major facts of what he/she just said, they can be considered a reliable witness.  One of my witnesses, in her testimony, assumed the majority of what happened at the end of the incident and had to tell the jury that she assumed some of the fact.  I would consider her as an unrelilable witness.