As someone who has suffered from several severe depressive episodes I would like to answer some questions.
There is a difference between someone being a bit off / in a bad mood for a day or two and a clinical depression. The latter brings with it some crazy consequences. A depression emanates itself by changed chemistry in the brain. This can be induced by various things like constant stress (not only emotional stress but also from heavy workload, physical exhaustion etc.). Once the brain is off balance, everything goes downhill.
Running into a depression is usually something that takes some time and precursors. However, if there is no one that can see the warning signs, this can go one for a long time until the depression gets so severe, that the depressive person only feels suffering and the only "solution" that person can see is to end the suffering once and for all. One of the side effects is, that you try to act normal and downplay what is actually going on inside of you. It is pretty easy to act normal - actually much easier than telling someone how you actually feel. You might want to call it sociopathic behaviour. Once the depression is in charge of you, you need a lot of will power to be able to fight back (see the problem?).
As a depressive person the first step is to get to a point where you see for yourself, that something is not right and that you need to do something about it. Since psychological illnesses are somewhat frowned upon by society, this is quite a task.
If you are lucky enough to get through it, work hard and get to know your body, what signs there are to take note of, what things to change in your life to deal with certain situations that would usually throw you off again, that does not mean you are healed. It is a bit like alcoholism, because the changed chemistry in your brain leaves behind a lot of casualties. As with every severe illness there are ups and downs along the way, but afterwards things are not the same.
In my case, I always had a very good memory and could focus on complex things without problems. After having dealt with a heavy depression (10 years ago), I changed a lot of things in my life but occasionally come to a point where I need to be careful to not fall into a deep hole again. The good thing is, that I learned how to deal with that for the most part. However, my memory is pretty bad now and it uses up a lot of energy for me to focus on certain things. Especially in an environment with a lot of people, I often can not follow a conversation. That has never been an issue before, but illustrates that a depressive episode takes its toll on the brain.
Please feel free to ask me anything regarding how I felt or what it was like.
To conclude: The main problem of a depression is, that the change of state of a person can be very gradually and no one surrounding that person will notice (unless someone in the close surrounding has already dealt with a depression).
Kind regards
Frederik