There are two sources of facts about a legal matter: the client and persons other than the client. In most cases, the primary source of facts in a legal matter is the client. Factual information about a client's case is obtained in an initial client interview. When additional information is required, a subsequent, fact-gathering interview may be conducted.

When the case requires information from someone other than the client, the paralegal may conduct a fact-gathering interview. There are two main types of fact-gathering interviews. In some cases, you may need to interview someone other than the client — a witness to an accident, the recipient of a contract offer, or a family member with knowledge about a custody dispute. These witnesses are known as lay witnesses. In most cases, lay witnesses may not state opinions; they may only report facts.

Other cases involve witnesses with specific expertise in a particular field. Expert witnesses are allowed to offer opinions based on specific facts. They may have opinions about the value of a piece of property, the business interruption damages suffered after a fire, or the effects of exposure to a certain chemical. Interviews with expert witnesses usually involve a combination of facts and opinions.