In-depth interviews can be performed face-to-face or over the telephone. The former is always most effective in that face-to-face discussions enable the skilled interviewer to understand what may lie behind the answers being supplied.
While relatively expensive there is no more effective way of gathering detailed and effective qualitative data.
Well done on the presentation and the work to date by the way. We feel confident that you have listened to us so far and will work with us and not against us which is a breath of fresh air!