Actors read a portion or the complete script before they are hired for a film and they are usually provided with a synopsis which would establish the nature of the character they are expected to play. They have the choice to take a screen test for a film which includes "bad language". If they don't like the contents of the script, they don't have to apply for the job. If the production team wants to employ a particular performer, the actor is also provided with a full or partial copy of the script and a character synopsis in advance. If the language offends, they can turn down the film. The production team may make changes to a script to suit an actor, but is not under any obligation to do so. If the script is changed during filming, which happens frequently, an actor could refuse to speak certain lines. However, the nature of the character is not usually changed, so the actor would know in advance what is expected of the role. Scriptwriters do not provide a character with atypical lines all of a sudden. If an actor has read the script or part of it, taken a screen test, been hired to play a role, and then refused to say their lines as written, they could be fired for breach of contract. I doubt many actors are presented with a line that includes swearing without knowing in advance how the character would behave and speak.

Some actors will refuse to say the lines a certain way and this may include not wanting to swear etc - this can really frustrate directors etc and can lead to problems (this is sometimes why you hear about relationship issues on a film set between a performer and director). Some directors have gotten a reputation for being very strict/harsh with their cast members, but as Jane Hamsher (producer on Natural Born Killers) states in her book "Killer Instinct", on the making of NBK, it is no use having a director who was hired for being known as 'a nice guy' because the actors will often run rings around him and treat him badly/take advantage - whereas a director being like a tyrant will hopefully get the job done. Seeing how some actors can have such a high opinion of themselves, sometimes they need to have their ego controlled. Ultimately they should deliver the lines that they signed on to do, but it often doesn't work as smoothly as that.

Sure they can. But they might not get that much work if they do. It's an ultra competative industry and I'm sure there are litterally millions of actors out there that will say anything you put in a script.