Now that you’ve gotten the first uncomfortable encounter completed, that is that first quick Hi/Bye conversation, you prepare for the next.
Whether it will be emails, phone calls, encounters at functions or by the way, there will be other meetings.
Don’t start throwing things. You know that you’ll run into your EX some place else, either alone or in company, and you have to prepare.
Remember; you know this person better than s/he knows her/himself.
You met, fell in love, you were part of this person’s life. You experienced their thoughts, their fears, their faults, their strengths. You fell out of love with this person. You know all the negatives and can list them.
You can certainly carry off chance encounters, answer emails, talk on the phone, even have lunch.
You can’t be deceived.