When you learn to play guitar sooner or later someone will ask you to accompany them while they sing. This isn’t as easy as just playing the song.You will need to practice with that person and make decisions together so you agree what you are doing. Tempo, how to start, how to end, stops and starts in between, dynamics; all these should be decided well before you go onstage.
Most singers will have a “best key” they sing a given song in. You might have to transpose the song to play it in a different key. You might have to speed up the tempo to accommodate the singer’s breath restrictions. You might have to slow it down to cater to their diction. You might have to play it in a different style. Some singers will work with you and let you be part of the decision-making process, while others will be more demanding and want complete control.
Get paid in advance for the latter.