Be serene, composed, polite or cordial, even in vigorous disagreement. Do not let your voice rise or fall but keep it modulated and reasonable.
If others listen then you may speak as loud as is necessary to be heard, no louder.
If a dialogue becomes of intense interest it should be deferred to a later date in a larger assembly room.
If you do lose composure you should apologise and defer the dialogue until you have had time to regain it.
There is no need to be angry in dialogue as the purpose of such a discussion is for both parties to leave the exchange enlightened by it, with a greater understanding or a clarified course of action.