Please note, I am not attempting to answer to what degree (or if at all) the viewer sins by watching a depiction of sin in the movies, only whether or not the actors themselves are guilty of sin.
But before we get into that, I think it is helpful to classify what types of sins can be depicted on film.
In the first place, we have sins that can be innocently simulated, that is, sins that can be imitated without actual commission. This is when an actor appears to be doing something sinful in the film that would indeed be sinful in real life, but which because of the situation, circumstances, etc. is not an actual sin. An example would be a bank robbery depicted in a movies. Were a person to do what is simulated in the movies in real life, it would be a serious sin; but because the actor is not holding up a bank but acting out a script, because the people he is robbing are not real employees of a bank but other actors who are "in on it," because the money he is stealing is not real money but fake, because the bank is not even a bank but a set and everything has been pre-determined, because nothing has really been stolen since everything is returned to its place when the shoot is over, we would be hard pressed to say the actor is guilty of committing an actual bank robbery. The same could be said for scenes when somebody is "killed" on screen - nobody is actually killed, nor is their anger or malice involved; these sorts of sins can be simulated without being actually committed. I think it would be easy to say then that actors can simulate these sorts of behaviors without being guilty of sin.
But this pertains only to the act itself - what about things relating to how the act is portrayed? Films can depict a bank robbery innocently enough, but is the bank robbery glamorized? Is it portrayed just matter-of-factly, or is it depicted negatively? The culpability of the actors in these cases depends on the degree to which the evil action they depict makes the viewer want to imitate it. But now we are talking about the effects of the actor on the viewer; I am more interested in looking at whether the actor sins by virtue of the act itself.
Besides sins that can be innocently simulated, we have another class: sins that cannot be simulated without commission. These are sins that you either commit or you don't, and if you attempt to simulate them, you commit them. The only way to not commit them is to not simulate them, or in other words, there is no difference between simulation and commission.
Let's take blasphemy. If the script calls for the actor to blaspheme, the actor must actually say the blasphemous words. If he says something different, he may laudably have avoided blasphemy, but he has not simulated it either. You either blaspheme or you don't. In what does the blasphemy consist? In the words spoken. Therefore, the actor, by the very fact that he reads the blasphemous lines in the script does actually commit blasphemy. If he wants to not commit blasphemy, he needs to not say those lines but alter them.
Isn't the culpability reduced because he is only playing a role? I would say it is increased rather than decreased. A man who blasphemes after hitting his thumb with a hammer is less culpable than one who readas a blasphemy from a script, for several reasons:
1) The scripted blasphemy is premeditated, rehearsed and done with absolute intentionality, unlike the latter case where the man simply blurts it out because of pain.
2) The scripted blasphemy is intended for an audience - not only this, but by the fact that the director/cast hopes the film will do well, they intention that their blasphemy have as large an audience as possible. Contrast this with the man who hits his hand, whose spontaneous blasphemy is meant for no one else and who would be embarassed to have his angry outburst published abroad.
In this particular instance (blasphemy), we can see that there is no difference between simulating blasphemy and committing blasphemy. In my opinion, actors should refuse to read blasphemous lines. There are so many easy ways to substitute other, less offensive things for a blasphemy. Besides, blasphemies are never really needed in films - there is no good reason why a blasphemy should ever be included in a movie. What does it really add to a film to have an actor say "GD"? Why couldn't they just say, "Dammit" without the Lord's name appended to the front?
Everything I said above with regards to blasphemy is also true of certain sexual depictions. I am not speaking here of pornographic acts, which we all know are wrong, but of other romantic actions - for example, laying in bed together semi-clothed, simulating sex acts, passionately kissing, etc. An actor cannot "pretend" to passionately kiss an actress without actually doing it. He can peck her on the cheek or do the kinds of fake kisses that they do in plays, which would be acceptable. But if they actually passionately kiss each other, then they are actually doing what they are simulating.
Does lust cease to burn because they are acting? More importantly, what are these actors and actresses guilty of if they are married in real life? It is my opinion that these actors are actually guilty of adultery. Take an example from Mel Gibson, whom we know to be married in real life (at least until recently). In the film Braveheart, if I remember correctly, Gibson does two romantic scenes with two different actresses, one in which both he and the actress appear nude. We know Gibson had a wife while he was filming these scenes. Does this cease to be infidelity because cameras are rolling? Ladies, would you feel comfortable if your husband did such things for money in front of a camera? The same is also true for Jim Caviezel in The Count of Monte Cristo.
We must ask ourselves - in what does adultery or adulterous actions consist? Do they not consist in doing romantic physical gestures with other persons while being married? Is there any definition of adultery that anybody can find that makes execptions for actors and actresses? If we are going to make exceptions, perhaps saying that Gibson's nude romance scene in Braveheart is acceptable because they are "only acting", then what of the porno actors who are in the same boat, albeit in a little bit deeper water? It seems to me that you must affirm both or esle acknowledge that both are sinful.
I'm sure there are other of these types of sins besides blasphemy and sexuality, but I mention these two because they are the two that most frequently sour movies I attempt to watch (which is not often). The key to which sins can be "safely" simulated and which cannot seems to be in whether the action is intrinsically evil or not - it is not always intrinsically evil to point a gun at somebody and shoot it - in this case, like if it is a pretend gun and a faked killing for part of a theatrical production. But it is always sinful to blaspheme; there is never a time or condition that can make blasphemy meritorious or even morally neutral. The same can be said about adulterous actions.
The saddest thing is that, from a strictly thematic viewpoint, sexuality and blasphemy never add to the plot of a film (unless the film itself is about blasphemy or sexuality, in which case it should have never been made). Think about it - was it necessary in order to move the plot of Braveheart along to see Mel Gibson nude with another woman? It the storyline of Back to the Future benefitted or deepend by having Marty say "GD"? How does it add to the themes of revenge and justice in the The Count of Monte Cristo to see a half-clothed Jim Caviezel two times simulating sex with another woman? In all of these cases, the films would have lost nothing by simply omitting these scenes; in fact, they would have gained much. Depicting sins like this add nothing to the plot and only make the film more difficult to watch for Christians who are embarassed and scandalized by this behavior.
These actions are even worse if the actors are Catholics.
These are just my opinions, but I think they are pretty well-thought out and synonymous with what we know about the culpability of our actions. If anybody has anything to add, I'd love to hear it. The really absurd thing is that, despite all this, there are plenty of perfectly good films that are getting a PG-13 rating for "Depicitons of Smoking"...