There are various things taken into account.
Firstly prices rise "generally" if a celeb features in the media. On top of this the news article is analysed to see what it's actually about. Depending on that process that price of the celeb is then adjusted further - either higher or lower.
A celeb being arrested or found guilty of something would be bad so their price is marked down. Conversely a celeb announcing good news would see a bigger rise take place than normal.
You will need to bear this in mind when it comes to investing. For example a celeb featuring in the news because they are reported to be involved in some shady dealings could mean they might be arrested. Whilst this would hit the headlines, if it were to happen their share price would drop drastically so investing in them might not be wise....
Traders actions affect prices but these actions are delayed so it's not possible for a "group" of traders to try and crash a celebs price and force everyone to sell their shares as they won't see an immediate impact on anything that happens. In the future this may change so that the window between trader actions and the price adjustments is narrower but for now it's staying as it. Yes, we aren't going to tell you what it is either so don't ask.