You cannot compel anyone to you to talk to you. Even if they do, their statements cannot be used as evidence if they are not present at trial. Interrogatories are written questions. They can only be propounded to parties, and the party has 30 days to file written answers. The written answers are under oath, so they are admissible at trial.
Questions are inexpensive to prepare.
You can also ask broader questions than you could ask in a deposition, including asking the party about the nature and support for their legal theories. You can often make the other party tell everything about their case or defense. The interrogatories are also prepared under the supervision of a lawyer, who can tilt the facts his or her own way. Request for production. You can serve a written request on any other party in the case to produce documents or other physical evidence.