Of course real witches can do magick spur of the moment! I practice magick all day long. But ask me when the last time I practice a planned ritual. Ostara, before that, Yule! Besides sabbats it isn't often.
When I dress, I choose colors that represent what energy I need/desire for the day. When I clean, I'm cleansing my home. When I bathe, I'm cleansing myself. When i cook I am mindful of ingredients that have meaning of prosperity, protection, or healing and stir it with energy so that my meal is not just a meal but an aid to my family. When I'm walking alone on a street or traveling in a storm, I cast a protection on myself and/or my vehicle. When my daughter gets hurt, I take care of it and put healing energy into it or hum a chant inaudibly. When I'm questioning something, I'll take whatever is available and divine with it, even as simply as tossing dirt on the pavement.
Magick is apart of a witches life! We live and breathe and experience magick everywhere. It is in us, not our tools or ritual circles, it is our mind, heart, and spirit and therefore goes where ever we go! All we need is focus, belief, and willpower.
My friend was in the hospital and performed a healing on himself on his hospital bed! No tools, just him. When I received a message that a friend of mine's wife was in labor and her and her unborn child were in danger of possibly dying due to extreme bleeding, I threw everything aside that I was doing, grabbed a candle, engraved Berkana into it with my finger nails, lit it and invoked Brigit, begging that the child and mother be safe and well. I didn't think, I just did it. No circle, no planning, a spontaneous spell. And to those wondering, yes, the mother and child are well. Thankfully neither had complications.
Magick does not need to be planned, it doesn't need a ritual, special tools, special words etc, it is everywhere and can be used at any time. There are no limits as to how to practice magick, only what it can do. We cannot cause physical manifestations to appear before everyone's eyes like the movies, but there is no reason why we cannot use magick anytime, anywhere, when we need it.