In a criminal trial, normally only a duly qualified “expert” is permitted to provide opinion evidence to the court. However, lay people, or non-experts are able to provide an opinion on matters that are part of everyday behavioural observation and don’t require special expertise.
Observations are limited to what the witness saw, heard, smelled, tasted or touched. An “expert”, however, may provide an opinion as to what a set of observations may mean. They are allowed to interpret the observations and provide the court with an analysis and professional opinion.
In many trials, the evidence comes down to the credibility and reliability of the testimony of the people who were at the scene of the alleged crime. Credibility refers to the truthfulness of a statement given by a witness. Reliability refers to the accuracy (or veracity) of that statement.