A couple of years ago I booked a nice role on a fairly big feature film. After we had shot for about six hours on the first night, and done most of my dialogue, the director walked up to me and introduced me to the stunt co-coordinator. Why was I meeting the stunt guy I wondered. I had not been notified about doing any stunts when I auditioned for the role, nor had my agent been told about any when he negotiated the contract.
This happens a lot - especially on films. The director or writer gets an idea to add a small stunt for your character during the shoot. Not a big, dangerous stunt like driving a car through a ring of fire, or jumping off a four-story building onto an air bag. I'm talking about things like jumping into a cold lake, running a sprint, or doing a somersault.
We actors hate to look like a wimp on a set. With the crew standing around, it's difficult to say "No" to the director. Especially when he or she applies pressure - "You can do this." "It'll look great!" "Come on - be team member, we won't let anything happen to you."
First, even the simplest of stunts should be done under the watchful eye of a stunt coordinator. He's the one who can tell if something could go wrong (unless of course he's really new to the job).
Too many inexperienced directors, especially on low-budget films, have no idea what the difference is between a big, obviously dangerous stunt, and one that seems mild, but on which you could get hurt performing. In fact, because professional stunt performers are hired to do the big stunts, it's usually on the small ones that we actors get hurt. Cuts, sprains, pinched nerves, muscle tears, lung damage (from smoke and dust), etc. are some of the things that can cause short or long-term health problems.
So, how should an actor handle it when he or she is asked, while shooting, to do something physical that isn't in the original script?
First, you need to have a clear understanding of what they want you to do. Sometimes they'll start small, and then ask you to do it faster, longer, higher, etc. If you agree to do it, but it starts becoming too much for you - say "That's it, no more."
Next, you need to have an honest idea of what you're physically in shape to do.
Lastly, you have to have the confidence to stand up for yourself. On many films and TV shows, you will be the only one to do that. I've seen actors get hurt - actually bleeding - and the director says "You okay for one more take?"
Last thought - be extra careful if the director springs this new stunt or physical action on you on the last day of shooting. If he thinks you might get hurt and not be able to work for a while, he may do this physical action at the end of your scenes, so that if you do get hurt, it won't interfere with the film's schedule. Does that sound like the director is a real a**hole? Yeah, but not necessarily intentionally - often he just wants to get whatever ideas he thinks are "cool" onto the film or video.
NOTE: This same issue comes up with nudity. It gets thrown at you on the set. Ultimately, we have to protect ourselves. That means reading our contracts, getting enough sleep, water, food, etc., and saying "No" when something doesn't feel right.
And what about the stunt I was asked to do on that film a few years ago? After talking it over with the stunt coordinator, I felt I could do it with no problems. However, I declined the stunt coordinator's help, so that I would be in control of the stunt - how hard I fell - instead of having him jerk me to the ground using a leash. It worked great, and I didn't get hurt. (But I still was not happy that it was sprung on me at the last minute.)