Excerpts from Robert's Rules of Order Newly Revised, 11th ed. (2011)

The parliamentarian is a consultant, commonly a professional, who advises the president and other officers, committees, and members on matters of
parliamentary procedure.  The parliamentarian's role during a meeting is purely an advisory and consultative one—since parliamentary law gives to the chair alone the power to rule on questions of order or to answer parliamentary inquiries. 

A small local organization should rarely require the services of a parliamentarian, unless it undertakes a general revision of its bylaws; but for large assemblies
and conventions or organizations where the transaction of business is apt to be complex, it is advisable to engage one.  Some state or national organizations find it advisable to employ a parliamentarian throughout the year to assist with any questions that may arise in interpreting bylaws and rules, or in connection with the work of the board and of officers or committees.  In such a case, the parliamentarian's duties extend beyond giving opinions to the presiding officer during meetings, and may include assisting in the planning and steering of business to be introduced. 

Appointment of the Parliamentarian.  If a parliamentarian is needed by an organization, the president should be free to appoint one in whom he has confidence.  The board or society must approve any fee that will be required, however.  If needed for only one meeting, a parliamentarian should be appointed as far as possible in advance of the meeting at which he is to serve, since his main work should be done outside the meeting. 

Duties of the Parliamentarian.  The president, knowing in advance the business to come before the assembly, should confer with the parliamentarian before the meetings open, and during recesses, in order to anticipate any problems that may arise and to avoid, as much as possible, frequent consultation during the meetings.  There is no set rule for the number of additional functions a parliamentarian may be asked to perform as a permanent appointee, such as teaching classes, holding office hours during conventions, and the like. 

During a meeting the work of the parliamentarian should be limited to giving advice to the chair and, when requested, to any other member.  It is also the duty of the parliamentarian—as inconspicuously as possible—to call the attention of the chair to any error in the proceedings that may affect the substantive rights of any member or may otherwise do harm.  There should be an understanding between the parliamentarian and the presiding officer that there will probably be occasions when it may be essential for the chair to listen to suggestions being made by the parliamentarian, even if it means momentarily not giving full attention to others or asking the assembly to stand at ease during the consultation.  This practice will enable the chair to be in a position to act promptly at the correct time and be fully informed.  In advising the chair, the parliamentarian should not wait until asked for advice—that may be too late.  An experienced parliamentarian will often see a problem developing and be able to head it off with a few words to the chair.  Only on the most involved matters should the parliamentarian actually be called upon to speak to the assembly; and the practice should be avoided if at all possible.  The parliamentarian should be assigned a seat next to the chair, so as to be convenient for consultation in a low voice, but the chair should try to avoid checking with the parliamentarian too frequently or too obviously.  After the parliamentarian has expressed an opinion on a point, the chair has the duty to make the final ruling and, in doing so, has the right to follow the advice of the parliamentarian or to disregard it.  But if the parliamentarian's advice on important procedural issues is habitually disregarded, he may find it necessary, at the end of the present engagement or session, to resign. 

A member of an assembly who acts as its parliamentarian has the same duty as the presiding officer to maintain a position of impartiality, and therefore does not make motions, participate in debate, or vote on any question except in the case of a ballot vote.  He does not cast a deciding vote, even if his vote would affect the result, since that would interfere with the chair's prerogative of doing so.  If a member feels that he cannot properly forgo these rights in order to serve as parliamentarian, he should not accept that position.  Unlike the presiding officer, the parliamentarian cannot temporarily relinquish his position in order to exercise such rights on a particular motion.[1]

Parliamentarian for a Convention

A key consultant in the preparation for a convention should be the parliamentarian, who should be engaged well in advance.  It is desirable that this person be a professional—the more seasoned in actual operating experience within organizations, the better.  Although he has the duty of giving parliamentary advice and opinions at convention meetings (where he should be seated next to the presiding officer), the parliamentarian's most important work may well be performed before the convention opens.  During the period of preparation and while the convention is in progress, he should serve as the principal adviser to the president, the officers, and the committee chairmen regarding management of the convention as it relates to the actual transaction of business.  The chairmen of the Credentials Committee, the Committee on Standing Rules, the Program Committee, the Resolutions Committee, the Elections Committee if there is one, and the standing committees who are to present business to the convention should all consult with the parliamentarian during this time; and it may be advisable that he should attend certain meetings of these committees.  The parliamentarian should always be present at the preconvention board meetings mentioned above.[2]

[1]  Robert’s Rules of Order Newly Revised (11th ed.), 2011, (RONR) pp.  465-467, [cross-references omitted]
[2] RONR, pp.  608-609.  [cross-references omitted]

See also:   Professional Ethics for Parliamentarians

                      John R. Berg's personal approach to Parliamentary Procedure

                     Articles by John R. Berg

What is a Parliamentarian?

Professional Registered Parliamentarian