now or never 立刻；机会难得；机不可失

【例句】
Excuse me, Sir. but I think it's only fair to tell you that it's now or never if you want me in your next picture .
我想为了公平起见现在最好告诉您：您现在可以选择我做你下一部片子的演员，否则您永远不会有这个机会了。

I felt it was now or never and went right up to him.
我感到机不可失，所以径直走到他面前。