Make an initial immediate assessment of the environment (including zeroing in on your suspect and what he is doing with his hands, especially if they are reaching into clothing like pockets or his waistband).
Remain focused on his hands while evaluating who he is in contact with and while searching for any potential proximal threats.
PRO TIP: If you see him before he sees you, you have a better chance at reading the entire situation and how he is involved in it; by keeping him under your careful and constant scrutiny will also put him on notice that you are a heads-up professional.
Assess your environment (having a cover man is optimal), but do not lose track of your subject, as that is when he will toss incriminating evidence, access weapons, attack you, or run away.
Do not announce your intention to make an arrest until you are in a position to follow through with it; giving an early indication to do so might prompt a spontaneous flight from you, or he may choose fight with you before you are set up to do so.
Stand at a 45° to the suspect when interviewing him (and in his rear quarter if you are the cover officer or are attempting to arrest him) – be mindful of the reactionary gap and his ability to attack you.
PRO TIP: About 85% of people are right-handed, so positioning yourself to capture his strong arm (initially just above his elbow) from his blind side makes tactical sense as it is difficult to ambush you when you are so positioned.
If you have a partner, strictly maintain the contact/cover roles: only the contact officer deals with the suspect as much as possible while the cover officer continuously assesses the subject and arrest scene for dangers and hazards.
Coordinate the initial arrest with your partner by using coded key words and subtle (facial/arm?) gestures.