Davening in shul with different nusach

How should an Ashkenazi conduct himself when attending a Sephardic shul, and vice versa? Should he daven from an Ashkenaz siddur, or use a sephardi siddur.

Is it any different for Nusach Ari/Sfard?

He should daven with his nusach from his type Of siddur. But he should follow the tzibur in all other aspects. For example. Stand when they stand , sit when they sit. That is, He should not publicize that he is davening from a different nusach. If they are saying something extra that he doesn’t have in his siddur he should join them w the opposite siddur.