The short answer
A lawyer generally cannot represent clients whose interests are directly adverse, or take a matter substantially related to a former client’s matter against that client, without informed written consent. Some conflicts cannot be waived.
Where it fits in your case
Before representation begins, and whenever circumstances change during the case.
What to expect
Current clients cannot be sued by their own lawyer, even in unrelated matters, without consent.
Former clients are protected in substantially related matters (Rule 1.9).
Imputation. One lawyer’s conflict often applies to the whole firm.
Waivers must explain the risks so consent is truly informed.
Disqualification in court is a separate question decided by the judge.
What we need from you
Names of all parties and affiliates, and prompt notice of new parties or relationships.
Common misconceptions
“Representing competitors is always a conflict.” Economic competition alone usually is not.
“Any conflict can be waived.” Some cannot.