Singers may find that they can produce a powerful sound during rehearsal but not during a performance or the reverse may occur. Adrenalin and tension can be a hinderance or a help depending on the character of the singer. In some it boosts the performance whilst others become stressed and try too hard.
When learning or practicing songs with a band it is easy to strain the voice by over singing. If a song is demanding, has difficult notes or requires powerful expression the singer should avoid performing at full belt on every run through. Use a lighter technique, hold back or hum the melody line until the accompanying musicians have a good grasp of the song, then have ONE run through at full efficiency and move on to the next song. You can always go back to the song later in the session, during the next band practice or during a vocal practice. The amount of time spent learning a song decreases in direct relation to the experience and proficiency of the performers. A professional singer who works with competent sight reading musicians is unlikely to require more than one or two rehearsals of a complete set whereas amateurs may need several hours of constant practice before one song is played to a reasonable standard.