There is a special attorney-client confidentiality rule (at least in most U.S. jurisdictions) that requires an attorney to keep confidential and protects with the attorney-client privilege, information disclosed when an attorney and client are in the process of evaluating whether they want to establish an attorney-client relationship.
If enough relevant information is exchanged, this information can also create a conflict of interest that would prevent the attorney from representing the opposing party in the same dispute.
The ethical rule in states that have adopted a version of the Model Rules of Professional Conduct is Rule 1.18:
Duties to Prospective Client
(a) A person who consults with a lawyer about the possibility of
forming a client-lawyer relationship with respect to a matter is a
(b) Even when no client-lawyer relationship ensues, a lawyer who has
learned information from a prospective client shall not use or reveal
that information, except as Rule 1.9 would permit with respect to
information of a former client.
(c) A lawyer subject to paragraph (b) shall not represent a client
with interests materially adverse to those of a prospective client in
the same or a substantially related matter if the lawyer received
information from the prospective client that could be significantly
harmful to the prospective client, except as provided in paragraph
(d). If a lawyer is disqualified from representation under this
paragraph, no lawyer in a firm with which that lawyer is associated
may knowingly undertake or continue representation in such a matter,
except as provided in paragraph (d).
(d) When the lawyer has received disqualifying information as defined
in paragraph (c), representation is permissible if:
(1) both the affected client and the prospective client have given
informed consent, confirmed in writing; or
(2) the lawyer who received the information took reasonable measures
to avoid exposure to more disqualifying information than was
reasonably necessary to determine whether to represent the prospective
(i) the disqualified lawyer is timely screened from any participation
in the matter and is apportioned no part of the fee therefrom; and
(ii) written notice is promptly given to the prospective client.