I would have to say that it depends on the eye witness. A human being has been proven to sometimes see things that are not actually there and believe things that never happened. The emotions that we have make us susceptible to having our perceptions skewed and out senses tricked. There's also the issue of someone lying for personal gain. That being said, I wouldn't say that eye witness testimony isn't reliable, but it can't be seen as infallible and should be able to hold up to a certain amount of scrutiny.
What is meant by "beyond a reasonable doubt"?
Beyond a reasonable doubt means that the proposed scenario is the only one that makes logical sense. That while some other explanations could be offered, those other explanations call for leaps of belief that an average intelligent person wouldn't make. If there is a logical argument to the contrary, if rational conclusions can lead to an alternative...then there is "reasonable doubt".
What are your impressions of the other characters?
With any movie, there are strong characters and weak characters. This movie has a rich collection of both. For instance the antagonist, a stubborn father, while having lots of personality, is a bitter man who has issues that spill over and affect his judgment. Conversely the elderly gentlemen that is the first to agree with Davis seems as though he has the wisdom that comes with his age. While he is the oldest, he is also the most observant as far as what went on in the trial. While most of the characters come out strong and explain why they change their votes, several do not. Of those characters, the advertising executive, the salesman, and the bigot all seem to show insecurity and make me question the strength of their character.
Is the main character sincere or does he have a hidden agenda?
Henry Fonda's character, Davis, doesn't have a hidden agenda from what you see in the movie. No details are brought to light about