I was thinking EXACTLY the same thing today - how weird that someone has started a thread about it

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Anyway, for me, Matt is definitely an old soul in a young body. He plays the Doctor this way. I can completely understand why, when they were auditioning for Doctor 11, they were going to go with someone older until Matt entered the room. Not only is he both old and young but he has an unworldliness about him. Or is that an 'other worldliness'? Am not sure.
As for his portrayal, not only does he bring the above to the role (all excellent attributes in my book) but also gives coherence and definition to our knowledge of this time traveller who looked into the time vortex as a young child and saw all of time and space before him.
You can 'see' that he sees the universe this way. He can see ALL of it, past, present and future all the time. Can you imagine the chaos in his mind? Matt's portrayal certainly gives me the feeling that he's trying to carry on being a 'normal' person, having conversations, travelling, attending weddings and dancing

but has to do that whilst trying to overcome or overpower or subdue, the 'chatter' in his head. His scattergun speech and his slightly ADHD approach to things, for me, is his way of getting on with what needs to be done, and saying what needs to be said whilst battling against a tide of sound and vision in his brain.
I know some people have said he's too manic or he speaks too quickly etc but for me, this is what makes him the Doctor. He IS this unworldly/other worldly person trying desperately to be something approximating human.
I think Matt's portrayal is spot on.