I rather not touch the gain. it will increase the cups of tea in post. Leave plenty headroom -12dBFS? and use 24bit depth if possible (permits a wider dynamic range capture than 16 bit)
I would pay attention to mic placement. Probably go for a dynamic mic if the acoustics of the room are not good, also they not respond to fast transients which may cause clipping.
You could also try to get the interviewee on you side by telling him cool stuff that they may not understand about acoustics and microphone characteristics, and ask him to control his/shes dynamics. Unfortunately they tend to forget everything you told them as soon as they start talking.:)