Monitoring means listening to the sound while recording. It should help the speaker follow the conversation, not force them to speak louder or chase their own delayed voice. Start at a comfortable listening level and make small changes.
Use two different controls on purpose
Recording gain changes the signal being captured. Headphone volume changes what the speaker hears. If the microphone level is safe but the guest sounds quiet in the headphones, check the listening mix before increasing recording gain.
Ask the guest to say one normal sentence and one slightly louder sentence. Set the recording input with those examples. Then adjust the headphone output so both voices are easy to follow. Repeat the short exchange after the adjustment to catch feedback, echo or an uncomfortable delay.
If you hear your own voice twice, identify the two monitoring paths. One may come from an audio interface and another from software. Keep the path that suits the setup instead of accepting a distracting double sound for the whole interview.
Check comfort during a break
A volume that seems useful for thirty seconds may feel tiring during a long conversation. Keep the level comfortable and take breaks when needed. If a listener needs a major volume change for one person, inspect that person's microphone setup rather than turning up everything.
At the end of the setup, save or note the working input and output choices. Ask everyone to avoid changing devices during the take. A simple repeatable setup gives you a better reference when a problem appears than a long chain of effects that nobody remembers configuring.