Ask "remember the future" questions
When asking someone to hypothesize about the future, ask that they imagine themselves at some specific future point in time and answer the question imagining that future.

Imagine the subject of your speculation has already occurred and it's some time past then. Reflect back on the event.
Often in the context of an interview or collaborative work session, we might ask someone to hypothesize about the future. We this might take the form of asking them what sorts of features would make their job easier, or what sorts of risks they thing a project has, or how they see a new tool being used in their jobs. These are often hard open ended questions to answer that often result in vague responses.
In Luke Hohman's Innovation Games, he describes the game "Remember the Future." The game is based on simple powerful idea based on the cognitive psychology concept of future perfect thinking. Luke's game asks potential consumers to imagine themselves at some time in the not too distant future. Then imagine they've been successfully using the product for some time period prior to that. Then the game asks them to write down what the product has done to make them happy.
In addition to getting more useful answers than the question "what features should we put in the product to make you happy?" this approach allows the questioner to control the timeframe as part of the context the question is asked in. The answer to a "remember the future" question will be very different if the timeframe is a month in the future or several years in the future. Hidden in the choice of timeframe is an element of prioritization. Given a short timeframe in the future, I've observed people giving answers that prioritize solving today's biggest problems. Given a longer timeframe, I've observed people considering larger long range issues or goals in their answer. Timeframe is an important constraint to better control the amount and type of response an open ended question like this can result in.
But all questions aren't "how did this product make you happy questions." Convert other types of questions where you're asking the answerer to hypothesize or speculate into "remember the future" questions.
To ask a "remember the future" question, you'll need to imagine the future a bit yourself. Imagine a timeframe most relevant to the information you're trying to obtain. This might be a few weeks after the next software release is installed, a month into the development of a new product, or tomorrow afternoon after handling a day's worth of customer service calls. Then choose a question that helps you identify your interviewee's speculations about the future.
For example to help identify the features included in the next release of the software consider asking this sort of remember the future question:
"Imagine today is two weeks after this software has been release. You've been using it effectively in your daily work. It's the end off this future day. Describe for me how your day has gone? How did you use the software? What did you most like about it? Did you have any challenges or problems?"
Use a "remember the future" question to capture negative information, fears or concerns. For example to capture potential risks on a project you might ask this sort of question:
"Imagine today is one month after the project has kicked off. We've been designing and developing software now for 4 weeks. It's the end of a long hard day, and things haven't gone well today. In fact things have gone poorly since the beginning. Describe the problems that have occurred in this project? What caused those problems?"
If you're trying to capture hard facts always avoid questions that ask the user to hypothesize or speculate. Consider this an interview objection where apposing council would shout "objection! – Calls for speculation!" However at times we need to obtain design ideas or get help setting priorities. This is where we need creative thinking. Using a remember the future question helps control the scope of and improve the quality of response from an open ended question that encourages creative speculation.