"Startup pressure can bring out the true character of a person. The problem though is that you may not know the true character of someone until startup pressure is applied -- even if that someone is your best friend."

This is true enough, but it need not be 'startup pressure' that reveals character. Many life situations can apply the same or greater quantum of pressure. So if you are going to do stressful startups with a friend, see if you have gone through hell before and come out on the other side still friends.

Tangentially life crises are very revealing of who your true friends are. Likewise romantic partners. When you are going through hell, just look around to see who is still there, and who ran for the hills.