Immortalpig - Statistically if someone is going to kill themselves, they will do it, they won't stick around to be talked down. If they are sticking around, what they want is attention. Furthermore, jumping is a particularly lethal means, if someone is jumping then they are probably well aware of the consequences.
So yeah, you don't have to try talk them down, but you probably should try, and if it succeeds, then they probably didn't really intend to do it in the first place. If they do go through with it, then there was probably nothing you could have done anyway.
Pidgenere - Immortal explained it better then I could.
I would most likely just try to call for help. There would be nothing I could do. Like immortal mentioned, if they really want to, they aren't going to wait around on me to talk to them.
This thread is really interesting... not because of the responses but rather seeing just how miss-educated American Citizens are.
A major factor in suicide cases is loneliness, just by talking to someone and asking them a simple question like "How are you feeling?" could go a long way. I'm sure everyone in this thread is capable of talking someone out of suicide and its really worrying that people still believe in myths like "There is nothing you can do" and "They want to die".
The truth is that the majority of people who say they want to end there life, do not, and just talking can help greatly.
Sources: http://www.samaritans.org/how-we-can...-about-suicide
http://www.mentalhealthamerica.net/suicide