A Love Triangle
Lacy (Scott Clarkson) is an obstacle to the Prince's desire: Margaret (Julian DeZotti).
Prince Edward (Paul Hopkins) with his dagger drawn.
Obstacles are crucially important to acting since without resistance to a character's will there is no conflict. Characters in drama struggle to overcome their obstacles in pursuit of their objectives. While characters usually have one clear objective, they often have many obstacles in their way.
The text tells us that the prince enters with his "poinard in his hand" and it seems that the Prince intends to kill Lacy and thus remove the major obstacle to his objective. Lacy and Margaret are defenceless, so in principle he could simply kill Lacy and carry Margaret away, but he doesn't do that. Why not? This may seem like a silly question but it is an important one. The actor has to decide what is stopping him from simply acting on his desire. What further obstacles are in his way?
In this instance we can rely on common sense and information from the given circumstances of the scene. The two most important obstacles to Edward here are the fact that:
1. Killing without a trial is illegal and dangerous even for a Prince especially when the victim is a powerful Earl.
2. Lacy was Edward's close friend.