What is the conflict of interest principle between lawyers and clients?

The classic statement of the conflict of interest principle between lawyers and clients is found in MacDonald Estate v. Martin, 1990 CanLII 32 (SCC), [1990] 3 S.C.R. 1235. Sopinka J. stated at para. 48-50: Typically, these cases require two questions to be answered: (1) Did the lawyer receive confidential information attributable to a solicitor and client relationship relevant to the matter at hand? (2) Is there a risk that it will be used to the prejudice of the client? … In my opinion, once it is shown by the client that there existed a previous relationship which is sufficiently related to the retainer from which it is sought to remove the solicitor, the court should infer that confidential information was imparted unless the solicitor satisfies the court that no information was imparted which could be relevant. This will be a difficult burden to discharge. Not only must the court’s degree of satisfaction be such that it would withstand the scrutiny of the reasonably-informed member of the public that no such information passed, but the burden must be discharged without revealing the specifics of the privileged communication. … The second question is whether the confidential information will be misused. A lawyer who has relevant confidential information cannot act against his client or former client. In such a case the disqualification is automatic. No assurances or undertakings not to use the information will avail. The lawyer cannot compartmentalize his or her mind so as to screen out what has been gleaned from the client and what was acquired elsewhere. Furthermore, there would be a danger that the lawyer would avoid use of information acquired legitimately because it might be perceived to have come from the client.