Then you brainstorm on a solution based on the notes from the interviews.
Then you interview another 20 people and pitch the idea to them. At least half of them should seem excited about it. If not, you gotta either rethink the solution or the problem.
And if you can't find 40 people to interview, that's another problem altogether.
Good luck!
And world wide or not doesn't matter. I did all my interviews over Skype.
If you're having problem finding people to get on a free Skype call then it's a big red flag. You'll need to reconsider your positioning or your market. Because eventually you'll (presumably) be asking for money and that's even harder.