I mean what I say and shall never react from [go back on] my words.

我讲话是算数的，决不反复。

《新英汉大辞典》

You never know how he is going to react.

你根本不知道他会作何反应。

《牛津词典》

But if you react to a life situation in a different way, a new way that you've never done before, you're being creative.

但是如果你以一种不同的方式重新经历你的生活情景，一种你没有做过的全新的方式，你的创意就会产生。

youdao

As a close friend of the twins Erin Junkans says you always need to be alert because you never quite know how people are going to react or what they are going to say.

BBC: Living a conjoined life

But as I've said, backs are tricky and we never know how exactly they're going to react.

WSJ: Yankees INF Youkilis happy with back prognosis

That sounds suspiciously like the Fed's position before and after the stockmarket bust: sit back as the bubble grows, but react aggressively in the aftermath and never mind criticism that the low interest rates of a few years ago helped to stoke the housing bubble.

ECONOMIST: Economics focus