Disclaimer: Not mine.
In the beginning, rookies think everything is cops and robbers. This game they used to play when they were kids, only now it's large scale, and somewhat real.
I say somewhat because for them, being a cop hasn't sunk in yet. They're still looking at their new badge, thinking how shiny it looks. Looking at their uniforms and thinking how sharp they look.
But for them, the Job isn't a job yet. They've got this picture in their heads of going into buildings, guns blazing and dragging out the bad guy, the murderer, what have you. It's helping little old ladies cross the street. It's all good guys and bad guys.
At least, in the beginning.