Often when I watch new Tai Chi players, their efforts to reach out too far and step further create a kind of bizzare ballet full of tension. Unless you are experienced, flexible and moving in alignment, doing Tai Chi this way will not get you the benefits which you will experience from doing it in a softer, less muscular way.
The flow of the Chi or vital energy is greatly enhanced by relaxing/loosening the physical structures near the energy pathways or meridians. That means if your muscles, tendons etc are tensed to their limits, the chi flow will be minimized.
At the other extreme if you are too soft and slumped in your posture, this also hampers the flow of the Chi, which flows faster on a gently elongated spine and extremities.
To the untrained eye, seeing the difference is difficult and that is why when beginners study with a teacher who is experienced, flexible and moving in alignment and they try to imitate his/her postures, they may think that they are doing it correctly, but in fact they are not. They might as well be doing another kind of exercise because they will not get the internal benefits of doing Tai Chi.
In a culture where we push ourselves physically as much as we can (more is better) especially when doing a sport or exercise, the idea of moving with a softer body is somewhat foreign. So where do we net out? Creating a balance between hard and soft is the answer. My opinion is: if you have to go one way or the other, at first, err on the side of being too soft. At least then you will get some of the internal benefits that Tai Chi can bring.
The Chinese word for this is 'soong'. Shape without tension. If you lift your arm in a posture, only use the minimum amount of power to effect the shape. Reach out, but keep an unlocked elbow joint. Step out but only as far as your foot can reach without locking your kneecap, tensing your calf muscle or reaching too far with your heel or toe.
Be a bit lazy at first.. as your strength and flexibility improves you can then add more 'shape ' to your moves and still be doing Tai Chi.