Planning and doing should be two distinct processes. The best people in any business do that.
Sure, you hear quips and quotes about how you need to "think on your feet", or "have a reactive business model", but that doesn't imply multitasking.
Watch Steve Jobs speak at events like D8, where he doesn't have the privelege of an orchestrated pitch or a fanboy audience, and where he has to answer questions "on the fly". Watch as he controls the tempo and thinks about each question before answering it. That, is "thinking on the fly".
If you try to mix the two, all that you'll achieve is constant doubt, and muddled execution.