Just to expand a bit on what G.C. said about John Farnam and the, "Can I help you?" question. Excalibur has it exactly correct: John recommends this as an initial verbalization, and Excalibur's example of considering it a 21-foot threat sphere is valid. Couple this with Farnam's training that early verbalizations are always accompanied by his "interview stance": slightly bladed body position; off-hand not fully extended in a "stop" gesture, but raised at mid-chest and arm half-extended, palm and fingers up; gun-hand held near the edge of the cover garment, not touching or exposing anything yet, but ready to. The body posture is clearly a prepared one to an experienced eye (and don't assume your street felon isn't experienced, even if he looks young), and the tone used is a firm one.
The issue of using a question--beyond that of what witnesses might or might not hear and recall--deals with the OODA Loop. If you've been targeted, a plan is already in motion. What you want to do at this point is interrupt the potential assailant's thought pattern for a split second. Your goal is to pause him long enough for you to scan 360, identify possible cover or escape, and understand if you're dealing with additional assailants. At this stage you're having to react to his action, so you want to introduce a momentary hiccup in his pattern that allows you to Observe and Orient yourself, and Decide quickly about your options. You're playing catch-up.
A declarative statement actually doesn't work as well as a question. An unexpected question disrupts cognitive processing and introduces a subconscious reply/delay factor. As an example, if you're about to take a swing at me and I say, "Hold up, dude," not only is it unlikely to disrupt your cognitive process, but it might actually trigger the next step. On the other hand, if I say, "What's your name?" it's likely that, if even for a millisecond, "What'd he ask that for?" is going to flash in between your last thought and your next.
This is a tried and true tactic in human interactions. Think about the last time you haggled with a salesman over a new car. Odds are, at several points during the proceedings he used a question to regain control of the conversation, used that brief disruption in your cognitive processing.
When you say, "What do you want?" you really don't care what he wants, and you have no interest in hearing his response or engaging in conversation. You want to break his immediate OODA Loop so you can catch up and be more aware and prepared if things escalate.
Your pre-patterned follow-up might be, "Sorry, I can't help you," your off-hand now turned fully palm-forward in an unmistakable "stay back" gesture."
Deciding in advance on your next verbalizations, as Excalibur described, is the right thing to do. Mine are simpler than the ones he mentioned, but he'd be less ruffled and much calmer than me.
