Well, I think what people often miss here is that.
Being "Fear less." and being "Brave."
Are two very very different things.. if you are fearless, you have no fear to overcome, and then you can never be actually brave, only stupid.
Bravery is to have fear, but then overcome that fear to do what you need to do any-way, if you stand against impossible odds, knowing you have no chance in hell, but then go in with all you got and fight with ALL that you have any-ways.. then you are extremely brave.
And that's why Usopp all-ready is brave, that is exactly what he has done for the entire manga.
Even upon his first introduction, he stood ready to face the Cat pirates, even though he knew he was out-numbered, out-skilled and would probably die trying.. he would try any-way.
It was that spark of bravery that made Luffy warm up to him and decide that he wanted him in his crew, and it's the same scenario Usopp has faced over and over again.
Out-matched, smart enough to know that he is out-matched. Fights any-how. And that's why he was brave from the get-go and has only grown even braver over time.
Two things he wont stand down for, no matter what happens.
1; His Nakamas.
2; His principles.
If his Nakamas are in danger, or his principles are made fun of so his pride gets hurt, then he will get so determined that he can appear down-right fearless.
So well, the scenario that would at least make him appear fearless.
One of his nakama caught and tortured while the villain makes fun of Usopp's principles pissing him off, there you go.
Or if his opponent just for ones is at least on his own level! Then Usopp gladly fight as well.. he is terriefied because his opponents are freaking monsters so much stronger than him, his fights are usually very unfair towards him <_<