"Zen practice is the direct expression of our true nature.... The most important thing is to express your true nature in the simplest, most adequate way and to appreciate it in the smallest existence."
--Shunryu Suzuki, Zen Mind, Beginner's Mind
Seen from the outside, Zen seems to be focused on calmness, simplicity and repetition. With this perspective, we might conclude that only calm, simple, repetitive behaviors "are Zen," and we might assume that the end goal of the practice is to become calm, simple, and repetitive.
Experienced from the inside, Zen is more like an empty framework that gives you a structure in which to explore your life. Whether your initial practice of Zen appears to be calm and simple is irrelevant. To focus on the framework itself is a distraction from the important issues. It's like when kids play with the box a toy came in and forget about the toy.
If you meditate in the Zen style because you want to be "Zen," then you're playing with the packaging. If, instead, you meditate in the Zen style because you've discovered that it helps you study yourself directly and it helps you root out the obstacles to happiness, then you're using the toy. Whether you appear to be calm and simple on the outside is not the point.
For people who discover Zen in adulthood, it's important to recognize that all the emotional baggage from the past will have to be a part of their early Zen practice. It takes a while to sift through it all, so there is a period of time, perhaps a long time, where their practice of Zen does not look, superficially, like the practice of an old Zen master.
It takes time to let go of old habits and simplify thoughts. Coming into Zen after decades of social conditioning that promotes greed and competition, we have no idea how to express our "true nature in the simplest, most adequate way." The simplicity of the Zen tradition provides a structure in which to explore the move toward simplicity. Gradually, we can let go of complex thought patterns and defense mechanisms and discover that we haven't lost anything important; we've only removed the barriers to our own happiness.