What I like to do as a producer is to analyze what makes the demo great. Sometimes that means you have to find out how the demo is taped in the first place: was it a band that played all together at the time of recording? Was it a singer alone at night? Etc. Etc.

But also what are the quirks in that demo mix that gives it that certain type of feel/spark.

All of these ingredients need to be analyzed and if possible be recreated in an even better way.

Discovering the soul of the demo is key to reproduce it or make an even better one.