I would pretty much support ADMIRAL's sentiments and Elaine's (good choice of words

) and Fellwanderer's.
The way I look at it is this [a broad generalisation, I know, but bear with me] an employed plumber may earn between say £20 - £50k pa and when he finishes his day's jobs he goes home, has his dinner and then his time is his own until the next day. a CEO may earn between £200 - £800k (plus bonuses) but his/her working day may not end until they go to bed, probably some weekend work too, but good benefits. The Actor/Actress that has made it may earn £multi millions pa. But, and this is the crux, the 24hr media scrutiny, intrusive fans, anti social working hours etc....this is what they buy into when they start off on that career path. Now, how they choose to deal with it is up to them. I will gladly pay my plumber for a job well done, The CEO of BT gets my money whether I like it or not, and the actor/actress only gets my money if I like their product. It only makes good sense to be as nice and as kind as possible all the time or you are going to potentially lose revenue. Look at Mel Gibson. If you can't take the heat....
I know that is broad brush, but I expect actors to be polite at all times and adapt to the circumstances. Some are obviously better than others at doing this, but that's human nature.
I don't particularly like Russel Crowe as a celebrity, but he's put in some good performances over the years. Someone who I cannot warm to though is Meg Ryan, regardless of her screen performances I have not seen anything about her off-screen that is remotely endearing. As far as the OP is concerned I think I would need several instances of selfishness/bad behaviour/poor performance before throwing in the towel on someone that I had hitherto respected.
And if you think a little wider of the mark, do the same rules apply for footballers? We tend to villify players that don't hold up to our expectations off the pitch...makes me think actors have it too cushy sometimes.