A trades person goes to work with a bag of tools. An actors tool kit is their voice and their body.
If the trades persons bag of tools are cheap and substandard then their work will obviously be affected. If the actors tools – their voice and their body – are substandard and blocked then their performance will also be affected.
As an actor you must be free of blocks so you can experience the range of human emotions required within every individual scene.
Blocks are anything that are blocking your voice or your body from free and dynamic expression.
Blocks can be caused by a constricted voice, by a fear of failure, fear of making a fool of yourself by really letting go, stiffness in physicality due to nerves or a lack of control of your physical actions (over-emoting, for example.)
Therefore to create a powerful performance you must free your body and voice of anything that is blocking it. The only way to achieve this is by focusing on the blocked areas and freeing them up.
Once you have achieved this then your instrument – your voice and your body – will be free to express itself over the full range of human emotions.