Give us an example of when you have coped well under high pressure?

[For people who do not have any experience in the airline]:

Once, when I was in the shopping mall, I have seen a lady that fainted and a lot people, that were standing around her. When I approached, I got to know that the daughter of this lady was almost not speaking English, she couldn't explain to anybody what happened. I found out that they were from Spain. As I knew Spanish, I quickly find out that the lady had a funeral of her son few days earlier and that she did not eat anything since then. After the lady drunk a bit of sweet drink, she felt better, then ambulance arrived and took care of her. The lady thank me many times before she was taken to hospital.

[For people who has experience in the airline]:

It was a flight from London to Spain. The lady, that was sitting in the front part of the plane, became pale and was feeling bad, she called one of my colleagues in the front, however, that lady was not speaking English at all, neither her daughter.

In that moment I joined the rest of the team in the front to ask if they would need any help. The lady suddenly started to shake so I hold the head of the lady to avoid any injury and started to talk with her daughter, as I knew Spanish, about any symptoms and possibly illnesses or events that could be a reason. I got to know that that lady was coming back from the funerals of her son and that she was not eating at all since a long time.
First Aid was provided, public announcement for the doctor was made in Spanish, oxygen was provided.

As a result, passenger was recovered, a medical personal was found on board as well as on the ground ambulance was waiting for her after landing. Passenger and her daughter was thankful to us.

Here you need to remember one of the challenging situations that happened to you at work, previous occupation or just in your life ( Important here: if you were already working as a Flight Attendant it is essential to bring examples from the times you worked on board).
The best way to reply to « tell me about the time… » question is using the STAR method.

Listen carefully to the question and try to think of an event or situation.

S - Situation ( describe the situation or an event that happened to you);
T - Task ( describe the task that had to be completed );
A - Action (describe an action completed by you in order to resolve an issue);
R - Result (finish your answer with the result achieved);

The STAR method give you a guaranty of present structure in your answer, and will include the information the interviewer is looking for.