What is the difference between an expert witness and a fact witness?

The fundamental difference between an expert witness and a fact witness is generally as follows: 

A fact witness usually provides testimony regarding what has happened in a given situation – he testifies as someone who has seen the situation, been involved in the situation in some way, and/or has some personal knowledge of the situation. An expert witness generally provides testimony as to how or why the situation occurred – he has analyzed the situation and gives his opinion based on his knowledge, training and experience as to the causes of the situation. In giving his opinion, an expert witness may testify using facts related to causation to support his opinion. 

In brief: The fact witness testifies that an assault occurred; the expert witness testifies that the assault occurred because the conditions/environment (such as lighting, lack of guards, etc.) were such that the assault was able to occur.

By-
Jon C. Paul, CPP,
Court-recognized Expert Witness,
Greater Milwaukee Area,
United States of America.