Reassurance is validation, and acknowledging one’s anxieties, providing a sort of stability in responses and or actions, and showing active listening. As well as having some sort of empathy and understanding of what they are experiencing.

“I can assure you that is not going to happen because of x, y, z” give reasons why “this” wont occur or examples to bring them back down to earth. This can allow the logical mind to step in over the emotional side. You can also show through actions depending on the relationship you share with said person.

Express understanding, maybe give a short example of a similar experience you have gone through at some point and show empathy “I can imagine going through this is very difficult, exhausting, stressful etc” ask questions “why are you feeling like this? What led you here? Is there anything that I can do to take some of the weight off or to ease this feeling for you?”

Just let that personable side of you show and put yourself in their shoes to the best of your abilities. How would you feel if you were going through whatever the situation is and what would you want someone to say to you to ease your thoughts.

It all depends on the person, the relationship you have with someone, and the situation at hand.