Report your failure as early as possible. It is advisable to do it during the interview, if you decided that the work this organization does not suit you. If you decided after the interview, call the HR Department and explain that you will not be able to go to work on the appointed day. Not just come without warning because the user is counting on you. In addition, the HR Department will need additional time to find a new employee.
Do not take your rejection of the work as a tragedy. Sometimes people are shy, afraid to announce his decision, and even a long time, genuinely hating the company and hoping to quickly leave. If the conditions of work do not suit you, feel free to refuse. Nothing bad will happen, the employer will just find another employee. Adequate guidance is always sympathetic to such situations.
If possible, try to politely argue your refusal. Of course, it is not necessary to tell you that you did not like the hiring Manager, or what do you think the head is not too smart and adequate person. Correct rejection of work is not even close to you the ability to try again to get a job in the same company. The most polite and correct argument in this case is an invitation to another, more suitable position.
Try not to go into details and tell you about all the events that persuaded you to abandon this work. Let your conversation with the employer will be short. Be polite, apologize for the inconvenience and wish to source good day, and company prosperity. You can also Express regret about the fact that you have to abandon such interesting work. Don't overdo it: your speech should not turn into a flood of flattery.