A decision to disfellowship will be made because the JC feels like they HAVE to - they have no other choice. Either the person confesses/spills his guts or too many people in the congregation have witnessed the wrongdoing (meaning two, three or more people testify against the accused) or he has been previously reproved for the same 'sin'. Also, if the Coordinator puts pressure on the committee, he can sway the decision making process because of pre-existing bias.
If someone has an elder for a father/uncle/good friend and gets 'coached' about what to say, how to say it, etc., they usually avoid the big DF - UNLESS one or more of the situations mentioned exist.
If a person ever 'avoids' getting disfellowshipped, he will find himself under constant scrutiny (with a target on his back).
The closed door judicial commitee has no scriptural precedent for christians.