Any wedding guitarist worth their fee is going to be able to sing. Not only that, but the singing will be accessible and not too stylistic. This means your guests will recognize the song.
They will provide sufficient sound reinforcement so both the vocal melody and the guitar accompaniment will be heard easily by you and your guests in a way that frames the ceremony instead of creating a distraction.
The song lyrics will follow the theme you have chosen and will be a perfect compliment to your ceremony.
This means your musician should play Classical, Pop, Folk, Fingerstyle, even light jazz. A wedding guitar player is constantly adding new requests to make sure you get exactly what you want.
This is different from your favorite coffee shop guitarist who tends to learn whatever moves their sound and following forward.
Multiple styles are important to an acoustic guitar wedding accompaniment to create the impression that nothing is missing which might be the effect since only one instrument is providing the musical setting.
Guitar players can sometimes be a little full of themselves. The last place for this type of behavior is your wedding ceremony when it comes to the music. The day is yours and everything a guitarist does supports you and the moments you have to treasure this special time.