In my experience,and yes I have had a couple people who were very close to me do it, I did nothing as I didn't know their plan which is generally the case if someone is serious about it. If they are talking to you about it get them help. That is what they are asking for.

If they want to talk, then listen. If they do not want to talk - DO NOT force them to talk. One can push a talker into a doer. Meaning what we say can be all that is needed to push a person to go from thinking about it to actually attempting it. Give them the suicide hotline number: 1-800-SUICIDE or 1-800-273-TALK For Teens: 1-800-367-7287 If the person is there on the bridge, call the emergency number (USA 911) and let the EMT/Police deal with it. If they have already taken drugs - call for an ambulance. If they are holding a gun, call the police and step back. The reality is that many people are 'just depressed' - they contemplate suicide but will not do it - unless they are pushed - like having an innocent conversation about the merits of death being pushed to an actual attempt because someone called the police in. If this is a terminal patient (cancer, aids, etc) where pain and suffering will proceed a certain death, then I strongly suggest being supportive, of listening and explaining that no you can not assist them because the law will tag you as a murder. Most of us are not experienced enough to deal with a potential suicide. I assure you that the person who is thinking suicide has given it far much more consideration than them just bringing it up to you now. They have most likely been thinking about it seriously for weeks, if not months. They have already weighed the pros and cons between life and the release of death. Most true suicides do not tell anyone prior to doing it. These are the ones with the highest success rates. Those who talk and then make an attempt usually make a 'poor attempt' or have made certain that there is someone there to call 911 for a stomach pump/resuscitation. in this case the act is a scream for help. If the person is only talking DO NOT tell them life is worth living unless you have a very good example or two handy. You can not say "Life is worth living" and the person say "Yeah? how?" and you say "I don't know" or "Its better than death" or some other lame 'reason' why life is better. The suicide is looking for an excuse - meaning she/he will consider what you say as 'proof' that life is good/bad/indifferent. If your proofs are not there or are lame, then you could make up the mind for the potential suicide. Listening without judgment is difficult to do. Most suicides who let it on what they are thinking are looking for someone to listen to them not argue with them, not tell the things they already know. One must be able to measure the situation - if this is all 'just talk' right now then you need to listen then find a professional that you can talk to (mental health care - psychologists are usually better since the are not quick to hand out drugs). Ask their guidance on what you should do now. If this is a situation where the suicide is armed and ready to do harm to self or others call 911 Emergency number for professional help.