There should be enough wiggle room to let a candidate's creativity shine through (which is in itself a useful signal), but ultimately by the end of the interview there will be a defined spec and designs of what needs to be built so the candidate can be prepared for their second interview. This interview is a candidate’s opportunity to ask questions they have for the company, and a company’s opportunity to get answers to any open questions they may still have about the candidate. It’s not unreasonable to come out of an interview to pressing work but it’s important to close this loop, try to make an official call by the end of the day (or early the next day if the interview took place in the afternoon) so the candidate will remember this process favorably no matter the result.