1) By reading the books, my perception of Doran Martell is of a person somewhat average in weight, who is affected by gout (swollen, chair-bound, with dark skin complexion. By comparing the perception I have received from the books to the visualisation of GoT, the only managed to get the skin complexion right. Based on the climatological depictions of Dorne, being at the very south of Westeros his clothing would have been very light and not of dark colours in order to minimise the effects of heat. Facial characteristics are bound by the fact of his illness and topography, namely a weathered face. Regarding posture, that would be no different compared to the average Dornishman. Regarding his personality, he appears to be calculating, and introvert, and in no need for caring and embracing the opinion of his peers. From his actions and inactions, regarding the murder of his sister Elia Martell, he appears to be pragmatic, as he did not openly challenge anyone who was involved in her and her childrens' killing, thinking of the greater good for the people of Dorne. His public stance on the killing does not demonstrate his private wanton revenge, but he is rational and knows that he can achieve more by plotting rather than warring. He does not try to protect himself, he does what he does for his people, in my opinion, but not in the way the others expect him to behave such as impulse.
2) Based on the book descriptions I would love to have seen for this role Toby Jones. Alexander Siddig as an appearance was very fancy compared to what he appears to be in the books. In one word "overaesthetic" for the role's age and troubles as depicted by GRRM.
3) I have seen most of these films and tv series, and I liked him a lot in Syriana. I did like his acting in GoT, and the portrayal of his character.
4) I have stumbled on a couple of interviews on youtube, and some articles, after I saw him on GoT. He appears to be down to earth, however since we are talking about actors, they can "act" their way and appear to be lovely, so from an interview I can never be sure about a person and his overall stance in life. I often ask myself, if it matters to me what kind of persons they are outside the set or the role, and I cannot find a convincing answer.