The writers admitted that they took some liberties, to make the show worth watching.
CSIs don't really investigate crimes, or take part in interviews. They just collect and analyse evidence. And they do it all wearing booties and "bunny suits" - white a full body covering (inc a hood) to prevent cross contamination.
The writers also cut out a lot of waiting around for results and fudged the science to keep things interesting. So the CSIs will often get results that would not be possible in real life. Leading to the "CSI Effect", where juries expect a mountain of forensic evidence for every crime.
