Interrogatories are a helpful tool for discovery. Interrogatories are written questions which focus on any information relevant to the case.When a Party May Serve Interrogatories. A party may propound interrogatories at any point up to thirty (30) days before the end of the discovery period. Tex. R. Interrogatories in Texas family law case are often used during custody fights to attempt to uncover what the other side is planning or thinking. A party must make a complete response to each interrogatory. For example, the Dallas County District Clerk and the Harris County District Clerk provide downloadable legal forms on their websites.