That's the thing about acting - it does have the feeling of downhill skiing. When it's really all going right, you know your lines, you know what's important to your character, you pick the strongest reactions possible to elements in the story. But then you let it all go and you're in the moment and stuff happens. It surprises you and it's super strong; it's like you're living life in a slightly heightened way in the time between "action" and "cut."