A decent guitarist may come along to the session with a few different guitars and a clear idea of what they want to record. Always remember not only is he/she the musician they are also the client. Just because your last set up worked ok doesn't mean it is always right. Listen to what they have to say you never know you might pick up a few tips and ideas. Often they are used to playing live but have little studio experience and they may want to hear the guitars built in pick up but an acoustic guitar will always sound better recorded with a micropone unless you want that particular clunky , one dimensional sound that you get from piezo guitar pick ups.

As with all instruments the sound you will get is altered in some way by the acoustics of the room you are in and if you haven't recorded in that space before get the guitarist to move around and listen to the guitar in different positions. Try other rooms and hear the difference playing in a larger room or a brighter room such as a tiled bathroom. Trust your ears and if the guitar sounds best in one place then set up there to record. Go with what the player wants to do because you must never forget that above all else the task of the recording engineer is to try and capture a great performance

Of course you can use any mic to record an acoustic guitar but the standard studio approach uses a combination of two quality cardiod condenser mics. The first is usually a small diaphragm cardioid like a Neumann KM84 or a Rode NT5 pointing at the point where the neck meets the body around the 12th fret and giving a bright clean sound. The second mic is generally a large diaphragm cardioid a little further away and getting a fuller rounder sound from the body of the guitar. By carefully combining these two signals you will give yourself the best chance of getting a good workable acoustic guitar sound in most situations.