Expression of Thoughts can be either Emotional or Conceptual. Among the Conceptual Expression of Thoughts, there are two variants 

  1. Through Writing 
  2. Through Speaking 

There are three neural circuits in Brain that express that you feel and the three neural circuits are in various stages of development in various individuals.

So 

  • Some are good speakers but poor writers 
  • Some are good writers but poor speakers
  • Some are Good Writers as well as Good Speakers 
  • Some are neither :) 

Same way, some people would like to read a mail to know the response. Some prefer a phone call. Why? It is because just like how you have various pathways for expression of thought, there are various input pathways too. Those whose neural circuits for symbols is stronger prefer a SMS or Mail. Those whose neural circuits for sounds is stronger prefer a phone call.

Now coming back to the problem, the neural circuits that deals with Speaking (From Brocas Area) is "relatively weaker" than the intellect and the neural circuits that deals with writing which are very very strong in some people. So they will find it difficult to talk.

However, I would like to clarify that their Speech Circuits are not really weak. They appear to be relatively weaker because their Thinking and Writing Circuits are extremely Strong. 

In other words, they think faster, their Hands are able to cope up with that speed, but their mouth is not yet coping up, and hence they stammer.

The Good News: A person with good intellect and good writing skills can easily develop his speech too. All it needs is self confidence (which I have given here ;) ) and practice (which they have to do.) 

Start Practising Today. Sorry, start now itself :) 

Best wishes for a successful career.