Not a police officer, but I imagine a lot of assertiveness in that profession is just from knowing your job ( probably applies to any profression really) through training and experience. I guess what I'm trying to say is you don't need to worry too much about being assertive, just make sure you have your shit together. A practical tip as a stutterer that works for me at least: When you do need to "assert yourself," make sure you maintain good breath support through your diaphragm, and focus on keeping your speech 'slow', controlled, and firm.