The first time I heard that I would be naked was when I got to the set that morning, but like many actors, I bit the bullet and said okay. After all, it was a funny scene and I was in pretty good shape, so why not? Which doesn't mean I wasn't feeling a little nervous about the whole thing.
So, eight hours later, when it's time to shoot the scene, I get into the bed with the actress playing my girlfriend, and remove my robe under the covers (I figured there's no reason to be running around naked any more than I had to). The camera rolls and the director calls "action!" My "wife" runs into the room with a shotgun and I leap out of bed and head for the bathroom. The director yells cut (no circumcision jokes, here) and informs me that they changed the scene so that I'll be wearing underwear. It would have been real nice if someone had told me about this change. I'm a little pissed off, but I get into my underwear and we shoot the scene again a few times.
The second time I was "sorta naked" was doing a sketch on a late night television show a few years ago. Again, nobody told me when I got the gig that I would be doing this, because they (the writers) only thought of it during the shoot. At first I said "no" but the producers convinced me to do it with two things - I would be wearing a "sock" (which is basically a special sock-like thing that goes over a man's private parts), and they would give me a large bonus (yes, that's "bonus"). So, I did the scene and it turned out really funny.
Okay, so what did I learn from these two experiences? Well, in the first one - a non- union indie, I learned that we actors are on our own (no union rules here), but that I should have asked if I could do the scene in my underwear when I was first told about it since I had not agreed to do a nude scene when I was hired. But I was new to professional acting and was intimidated. I didn't realize I had the power to say "no," especially since I had already shot a lot of my scenes and they sure as hell wouldn't have fired me and reshot every scene I was in.
As for a union project, like the TV sketch, they were in violation of the contract. SAG-AFTRA rules state that you must be informed BEFORE agreeing to do a project, that there will be nudity involved in your role. Again, this scene was the last one after several days of shooting, so I could have said no. And if it had been full nudity, I would have - but with the "sock" and yes, the extra money, I was cool with it.
If you don't feel comfortable doing something - and you haven't agreed to do it when you were hired (and that includes stunts, as well), then don't do it. However, if you agreed to do nudity when you were hired, and then changed your mind - the producers do have a right to be upset - and even fire you.
We actors need to feel proud of our work, so keep that in mind whenever you are faced with a tough decision before or after you're hired.