My school did Cosi fan tutte last semester. the Don Alfonso (the ringmaster who sets up the plot) was so wooden he looked like he was tied to a board. I mean he had exactly one pose, arms down by his side, a few inches from his waste, feet about shoulder width apart. He'd twist his body towards the person he was singing to. It was truly awful acting, fortunately Cosi fan tutte is a comedy and he at least looked comedically bad.

In May I saw two english operas (The Wandering Scholar and Savitri) performed by a professional company in Manhattan (a small company, think it was called 'little opera company' or something). The acting was superb, I practically wept over Savitri when she started crying for her dead husband.

Point is, good acting helps a lot ... bad acting can drag a performance down.