Consider the sentence "Marcus spoke as a manager". Imagine that Marcus was speaking at a company event, and he gave his speech as a manager, not as a coworker — as a representative of the company, not his private self. To emphasize this aspect, one might say "Marcus spoke in the capacity (or role) of manager". I am not sure if I master the English idiom perfectly, but I hope my point gets across.
Now, I would like to be able to add this emphasis in Latin, too. The best starting point would be an apposition, I believe:
Marcus dux locutus est.
Marcus spoke as a manager.
Dux might not be a good translation for "manager", but that is irrelevant. But how to form a more emphatic version? I imagine some of these might work:
Marcus in/e munere ducis locutus est.
Marcus in/ex officio ducis locutus est.
I prefer classical Latin if attested examples are available.