"Be formless...shapeless...like water. Now you put water into a cup, it becomes the cup. You put it into a teapot, it becomes the teapot. Now water can flow, or it can crash...be water, my friend." -- Bruce Lee
The above quote is a great example of "going with the flow". As actors, we've all heard about "being in the moment", a Zen-like state where we can react to our scene partner without any preconceived notion as to how we are going to respond. Trouble is, when we've been in class long enough to have learned a technique, we begin to over-prepare what we're going to do and how we're going to approach the scene. This takes a natural response and turns it into a kind of acting "choreography".
Choreography is nice when you're doing a dance number, or a fight scene. But choreographed acting looks robotic and lacks any ability to change or adjust as needed. When you preset all of your "beats" and "moments" in an audition, it turns you into a mechanical automaton and looks stilted on camera.
Lots of actors come from a stage background where this mechanical type of acting isn't as apparent. And since stage blocking has a kind of choreography to it as well, it reads okay. It's a lot like watching a kata performance in martial arts. For those of you who don't know, a kata is a pre-arranged routine of movement and technique performed solo against imaginary opponents. The pattern does not change and is the same every time you perform it. It's very similar to a fight choreography.
But when you spar in the martial arts, all prearranged movement goes out the window. You're required to respond to an attack that you don't know is coming, in real time. Improv, so to speak. While kata can teach you useful techniques, you have to be able to apply them in a live situation and adjust to the changing conditions.
So it is with good acting. Even when the director gives you a way to approach the material, when you get on set or on stage, the other actors in the scene are going to influence how you perform. Your preparation as an actor should be complete, but you need to be able to "let it go" when the cameras roll or the lights go up.
Improv and sparring are so similar, I often reference both of them when teaching. I'll teach actors about sparring and being able to respond without over-thinking, and my karate students learn about Improv and "being in the moment".
So, preparation and good form are nice. But remember, in order to apply them, you need to be able to "do it live".