Look for other ways to make the singer comfortable. A good place to start is with the vibe of the studio. Many recording spaces feel very sterile or clinical, and some singers are sensitive to that. Lighting, temperature, air quality, and decor can all affect a singer's state of mind. Although some can deal with any environment, others need candles, tapestries, aromatherapy, and pictures of their childhood home in the booth. You don't have to hire a feng shui consultant, but within reason, try to accommodate a singer's requests. Believe me, it will pay off with fewer takes, fewer punches, and a more inspired performance.
Be wary of making the vocalist feel as though he or she is in a fishbowl. Although some singers don't care who is in the control room or peering through the glass, others get uptight.
I did a project a few years ago with a singer who was an old friend with little studio experience. She was nervous to begin with, and the presence of band members in the control room watching through the glass made her tense and caused a tiff. I cleared the room and spent a few minutes chatting with her to calm her down and take the pressure off. I lowered the lights and said I just wanted her to feel comfortable and sing her song on her own terms.
In the end we got a great vocal on tape, and it had little to do with the AKG C 12 in front of her; she was able to get in the mood and immerse herself in the song and the performance. When the rest of the band heard the playback, they were surprised by how emotive her performance was, and they realized how much their presence had intimidated her during the earlier takes.