ONE OF THE MOST VITAL FUNCTIONS OF A UNION STEWARD is to prevent management from intimidating employees. Nowhere is this more important than in closed-door meetings when supervisors or guards, often trained in interrogation techniques, attempt to coerce employees into confessing to wrongdoing.
The rights of employees to the presence of union representatives during investigatory interviews was announced by the U.S. Supreme Court in 1975 in NLRB v. J. Weingarten, Inc. Since that case involved a clerk being investigated by the Weingarten Company, these rights have become known as Weingarten rights.
Unions should encourage workers to assert their Weingarten rights. The presence of a steward can help in many ways. For example:

The steward can help a fearful or inarticulate employee explain what happened.

The steward can raise extenuating factors.

The steward can advise an employee against blindly denying everything, (hereby giving the appearance of dishonesty and guilt.

The steward can help prevent an employee from making fatal admissions.

The steward can stop an employee from losing his or her temper, and perhaps getting fired for insubordination.

The steward can serve as a witness to prevent supervisors from giving a false account of the conversation.

Weingarten Rules
Under the Supreme Court’s Weingarten decision, when an investigatory interview occurs, the following rules apply:
Rule 1. The employee must make a clear request for union representation before or during the interview. The employee cannot be punished for making this request.
Rule 2. After the employee makes the request, the employer must choose from among three options. The employer must either:
a. Grant the request and delay questioning until the union representative arrives and has a chance to contact privately with the employee; or
b. Deny the request and end the interview immediately; or
c. Give the employee a choice of: (1) having the interview without representation or (2) ending the interview.
Rule 3. If the employer denies the request for union representation, and continues to ask questions, it Commits an unfair labor practice and the employee has the right to refuse to answer. The employer may not discipline the employee for such a refusal.