You do have to invoke it.
Being an expert on law enforcement (I've seen every episode of Cops!) they always tell the guy, just cooperate and things will go better. I saw one the other day where the officer flat out said the guy wouldn't go to jail if he cooperated. He did and they hauled his but to jail.
Cops always say, "Mind if we search your vehicle?" Then when you say no, they say, "What do you have to hide?" Never agree to a search, they're going to search if they want to but if you agree to it, you lose out in court if you try to claim they didn't have probable cause.
Many years ago, I read a book called, "Patrol Procedure". In it, they explained it was OK to lie to get information from a suspect. Nice.
Officer friendly was laid to rest many years ago. Now we have officer roided out punk, thug. If you have to talk to cops, always talk to the oldest one in the bunch. They don't need to project the tough guy image and they've seen it all. Just watch Cops! and see how the young cops react to people compared to seasoned cops.
One final question, when did police forces allow their officers to have tats that look like they were in prison for twenty years? You want a tat, fine, don't expect many offers for any CEO positions.
/johnny