You don't need to pull someone over and do something which is illegal for a civilian. If you just modify a car to look like a police car and driver around with it, then that is likely enough. Similar, even if you only walk around in a police uniform with a fake badge and don't do anything more, it will likely end up badly.

Context matter, which is why intent and result are part of the equation. If people are being fooled into thinking someone is a police, and someone had the intent to fool others (maybe with the intention to create false association), then that is likely fully enough to get a person charged with impersonating a police officer.