Customers are unreliable about the future and reliable about the past. Ask about what happened, not about what they would like, and the answers stop being polite guesswork.
Put the draft questions on a board and let the whole team react. It surfaces the leading questions and the ones three people wrote in different words.
'Would you use this?' gets a yes from almost everyone. 'What did you do the last time this came up?' gets the truth.
After each interview, drop the quotes that surprised you onto a shared board. Patterns across interviews show up in the reactions long before anyone writes a report.
You are usually close when new interviews stop surprising you — often after five to eight conversations with a similar type of customer. Collecting the takeaways on a shared board makes that point easy to see.
Have someone who is not running the interview review the question list first. Anything containing your solution, or answerable with yes, is usually leading.