Will asking someone if they are suicidal make it more likely?

No. Reviews of the research consistently find that asking directly does not increase suicidal thoughts and often reduces distress. The idea that it plants the thought is one of the most persistent and most harmful myths in this field, because it stops the conversations that help.

What should I say when someone tells me they are suicidal?

Thank them for telling you, stay calm, and stay present. You are not expected to fix it. Ask whether they are safe right now, listen without arguing them out of it, and offer to contact a service together rather than on their behalf.

Should I promise to keep it secret?

Do not promise absolute secrecy, because you may need help to keep them safe. Be honest instead: say you will not tell people casually, and that if you are seriously worried you will want to involve someone who can help, and that you will try to do that with them rather than behind their back.

Supporting someone who is suicidal

You do not need training, and you do not need the right words. Most of what helps is far more ordinary than people expect.

Ask directly

Ask the actual question: "Are you thinking about suicide?" Not "you're not thinking of doing anything silly, are you?", which tells the person you cannot handle a yes.

Asking does not put the idea in anyone's head. That belief is widespread, it is wrong, and it is responsible for an enormous number of conversations that never happened. Reviews of the research find that asking directly does not increase suicidal thoughts and frequently reduces distress — being asked plainly by someone who is not frightened of the answer is, for many people, a relief.

Ask when you can give it time. Ask somewhere you will not be interrupted. Then wait.

What helps

Staying. Sitting with someone in silence is doing something, and it is often the thing people remember afterwards.
Taking it at face value. Not "you don't mean that", not "you've got so much to live for" — however true it feels to you, it lands as an argument, and an argument is something they now have to lose.
Asking what would help right now. Sometimes the answer is a lift somewhere, or a meal, or someone to make a phone call they cannot face.
Being concrete. "I'll come round at seven" beats "let me know if you need anything", which quietly puts the work back on the person least able to do it.
Following up. Two days later. A week later. The people who help most are the ones who come back when the crisis has stopped being interesting.

What does not help

Panicking visibly, which teaches them not to tell you next time.
Listing reasons to live, which sounds like being told they are wrong.
Making it about you — "how could you do this to me?"
Taking over. Doing things with them keeps their agency; doing things to them removes it.
Promising absolute secrecy. See below.

On keeping it secret

Do not promise you will tell nobody, because you may need help to keep them safe and a broken promise costs you the trust you will need later. Be honest instead:

"I'm not going to tell people casually. But if I'm seriously worried about you I'm going to want help, and I'd much rather do that with you than behind your back."

Make a plan together

If they are willing, sit down and fill in a safety plan together. Being on someone's plan is a genuinely useful thing to be, and it makes the next hard night far easier for both of you — they already know they can ring you, because it is written down.

Look after yourself

Supporting someone suicidal is exhausting, frightening and often lonely, and the people doing it are almost always the last to be offered anything. You are allowed to find it hard. You are allowed to need your own support, and most of the helplines on this site will talk to you about someone else — you do not have to be the person in crisis to call.

You are also not responsible for another adult's survival. You can do everything right and still not be able to control the outcome. That is not a failure of yours.