Adult Mental Health Help

At what point would you get professional help for an adult? Not counting when someone is suicidal, but if they're behavior is getting strange, when is it time to intervene? And in what way would you intervene?

*someone asked why this was controversial and then deleted their comment. I figured this was a question for CC bc it can be considered controversial to get an adult mental health help against their will. It may be surprising to learn how hard it can be to intervene. Some people think you should just leave them alone, and it's hard to know when you need to step in.