This is a very rough game - there are two sides against you who are willing to use many tricks to stop you winning. They have a lot to lose if you win. They can lose reputations, their job prospects - and sometimes even their jobs. These two opponents are the police and the judges. You could not choose harder opponents. Each has the law on their side - they can use it against you - and you can bet that they will if they get the chance. So you must not break the law during any of your investigation because the other side will take great delight in making you pay for it - and in the process your entire investigation will be stopped, forever.
The main laws you must not break are:
Contempt of court. - this means, in the main, the legal documents you have from the trial ( or elsewhere) Many of these have the word "restricted" on the top of them. The convict in the case has permission to look at them, but others do not have - unless they have written permission from the convict. So make sure you have written permission from the convict to act for him or her and to see all the documentation on the case. When you get the documents, do not go showing them to others - because to do so is probably contempt of court. Contempt of court can be punished by sending you to jail.
Harassment. A basic rule of thumb on this is " two approaches OK, three is harassment". So if you find someone who can help you, but who is reluctant to do so and get involved in a court case, be very careful to make sure you do not harass them. If they go to the police your approach is finished.
Approaching witnesses who appeared at the trial is out. Do not do it. You will, no doubt, be asking them to change their testimony in some way. The police will pay the witness a visit and point out that changing testimony like that is perjury - and merits perhaps 9 years in jail, with children taken into care etc etc. You have no idea what lies some people will tell sometimes in order to get themselves back in the good books of the police. After all, the police have powers with which they can harass people such as those you would have liked to talk to. So you might find yourself charged with a lot worse than harassment.
Trespass. If someone asks you to leave their property, you must do so. This is because of the same reasons as listed under harassment. On the other hand, remember "tourist rights" If a Japanese tourist can go somewhere, take pictures etc, then so can you. Someone may say that you are working on a "journalistic enterprise" and you are going to breach someone's privacy by taking pictures or simply intruding - unlike a tourist. But you are not doing that are you?
Defamation, or libel and slander, is when you say (or write) something to someone about another person which lowers the reputation of that person in the mind of the person hearing what you are saying. So if you go around witnesses slagging off a policeman and so ruining his reputation, he may well sue you for defamation. And since the other team in this game is the judges, you are almost bound to lose the case. They will all go off laughing to the bank to get your money. Laughing - because they have won the game. They don't need the money, they have plenty of that - from the honest citizens who pay their taxes.
If you are visiting someone to see if they have any recollection that might help your case, it is often useful to carry a mini-cassette recorder or an MP3recorder in a top pocket somewhere. This is not to "bug" them particularly - but to record what you are saying. This is because once the police find out you have talked to that person they may well pay a visit - and then you find that the witness has forgotten a lot of things. You did not say what you were asking questions for, you slagged off the police. You said the judges were all rogues, you said a lot of things that might get you in a lot of hot water; worst - you lied your head off about the details of the case in an effort to get them to tell you what you wanted to hear. It's amazing what memories some people can have. But if you have a recording of what you actually said, it is amazing how they change their tune yet again.