Isn't trust-building a process? Like walking out on the ice, testing it with your foot? The word "trust" covers so many different areas?

I used to say of one of my dearest friends (and it was literally true): "You would trust him with your life, but you wouldn't trust him to return a library book on time."

What exactly does GP mean by wanting you to trust him (yeah, I get the dark alley thing, but I want a literal answer not a metaphorical one)?