I like to think I'm okay. It can depend a lot on the director, DP, the script, other actors, and even the lighting. I definitely benefit from more prep time and feedback from a communicative director who has a clear idea what he wants.
I wouldn't say it's easy: there's a lot of pressure; on a non-SAG film, 10-hour+ shooting days and 6-day weeks are standard, and a lot of roles can be quite physically demanding. To get what you see on screen, you typically have to do the same sequence of actions over and over again, not necessarily in order, and what really happens may be a lot different from what it looks like after it's been edited. (e.g., in the movie world, you're having an angry confrontation...except that the rest of the people in the scene were filmed somewhere else a week before, or maybe they won't be filmed until next week.)