fiedgreentomato - that is a common comment - and it has a very simple basis.
Anyone who suffers from depression learns to hide the symptoms very quickly and well - because the attention and interest is far too painful. It's not so much being seen 'naked' - even by family and loved ones, it is as if you are being seen with no skin on your soul - the exposure is as deep and painful as that.
So yoou develop a mask of 'normality' and the deeper the depression goes, the thicker the mask becomes until being 'normal' becomes a reflex action that you put ion place when you open your eyes in a morning.
That is why so many suicides are seen as 'the last person i would have thought of ...' - that simply means their defences worked - and you didn't see the 'real' person because they didn't want you to see them, and they made very sure that you didn't.
That is why people who feel guilt after the suicide of a loved one or friend, should not feel so.
There was nothing you could have done, your lost soul was beyond your help, and beyond finding it even if it was there. You could not have saved them, you did no wrong by not seeing past their 'image' - you do not have the skills to see, so don't blame yoursef at all.
If i ever succomb, and you never know, I hope no-one feels I was brave, or cowardly, i was neither of those - i was just - finally - desparate enough.