I think that Eccleson didn't realise what he was getting himself into. Apparently he is quite a shy person (not sure why he became an actor then?). I saw him on Jonathan Ross last Friday and he looked very nervous when he was being interviewed.
He then looked pretty shocked when Ross showed him a to figure of the Doctor which looked like Eccleson. For someone who does not like publicity, having toy's in their image is not really what they want!
Most actors are usually very shy people in their private lives...it's part of the mental make-up that drives an actor to act...to be someone else. That has nothing to do with it.
As I've said prior this, there is simply no way Eccleston didn't know what to expect from this role, so his excuse is just that, an excuse sent out (a rather overused one at that) to cover for the real issues in order not to harm the product (in this case Dr. Who's big return).
As for how they will replace him, I couldn't say, but to actually regenerate him completely, IMO, would be a waste, when they can replace him without doing so.
If the finale ends as they say, then that tells me they wrote it in such a way as to regenerate him on purpose...which stinks of inside problems that were going on the whole time...so they wrote themselves an out. (If that is true.)
Thewlis would be a great choice, as well, and again would NOT need a regeneration to take the role...it's called, use your imagination.
As to the Romana "regenerations" in the beginning of Destiny of the Daleks...much speculation has been out there as to just what that was. It is clear that Time Lords can regenerate at will and can be forced to regenerate (as was Troughton to Pertwee). However, what Romana was doing was not actually regenerrating into all of those forms each time she appeared. The best explanation for that was that she was projecting mental images into the Doctor's mind of what she might look like (Time Lords are telepathic to other TIme Lords after all). Meanwhile, the whole time, she had already regenerated into Lala Ward, since she first came out as her and last came out as her, the intervening "appearances" being to convince the Doctor of a decision she had already made.
I too believe that if they go the big screen as they are still wanting to do, that they should follow Paul McGann's doctor, since he showed a great deal of potential as the Doctor (though the US movie had its obvious problems). I've always felt his Doctor was shafted.
No...there is more behind the Eccleston exodus than they are telling and than we will most likely ever be told.
I just hope they don't waste a regeneration over this when it's really not necessary (especially if Tennant gets the role, since he so closely resembles Eccleston). I little thought and imagination goes a long way and keeps the tenth regeneration still years away.
CE
), but he characterized the spirit of the character perfectly, IMHO. 
