We record in a very small room (our desks surround a single backdrop and lighting setup), and when we bring people in for interviews they all tend to speak quietly because it's a small space. Is there anything we can do to make the space more comfortable for them to naturally speak louder? For some reason asking them to speak a little louder always sounds forced.

Thoughts?

Get them comfortable by talking with them at the level you would have them talk at.
Tell them to talk just like they would to their spouse or friend carrying on a normal conversation.

It could be because of slight nerves, don't rush 'em into it .. beforehand try a few jokes and a couple of shots could help.

Cheers.