So let's assume time travel is out of the question. How can we ensure that a vocalist has a good chance of nailing a recording before it starts? This is where the other sides of being a recording engineer starts to come in handy. Your technical and musical talent is expected, but your ability to be a psychologist, therapist, interior designer, and a billion other things are just as important if you want to capture that "perfect" recording.
Whenever possible, I encourage artists to arrive for a session 30-60 minutes before the actual session time. This gives us a chance to chat about the session goals, but more importantly it becomes a chance to see how the person is vibing that day. Are they exhausted from work? If so, maybe I'll mic closer than normal and turn the instrumentation down in the mix while tracking so they don't burn out from yelling to be heard. Maybe we should record with the vocalist sitting on a stool too. Let's take another example. Your artist has been arguing with someone. It might be time for a few minutes of consoling, and supporting your client so their self esteem is in the right place to get this record done. If it's an angry or aggressive record, maybe it's time to side with them on the issue and hype them up. While recording, be sure to maintain a heightened level of patience and watch your tone when making suggestions if their in a mood that warrants it. Artists are usually emotional people, so if you take the extra time to make sure those emotions are in the right place before a session, you'll establish trust and make a better record.
If you get a good recording through proper mic technique, half of the battle is already won. With that in mind, it's essential to physically watch and in some cases guide how a vocalist approaches the mic when recording. Often artists that are new to recording will move a lot, or not make a note of where they were standing in between takes. When that happens, you get a vocal with a lot of volume inconsistencies that have to get fixed with compression. To combat this, once we've established a good standing spot in reference to the mic, I place two pieces of tape on the floor, right by the artist's toes. This is now "home base". By telling a vocalist you can only record from "home base" you'll fix a lot of volume, proximity, and phase issues from the gate. Take some time and educate yourself on mic technique, even if it's just through trial and error. The more you can guide the vocal performance, the better. Just be positive and respectful when making your suggestions.
Empty the Room
Sometimes vocalists like bringing friends to the studio and there's nothing you can do about it. But whenever possible, I always encourage artists to only show up with people that are actively contributing to the session. Furthermore, when working in someone else's studio space, I do everything possible to make sure interns or assistants have set up and stocked ahead of time so there's very little foot traffic when the session starts. If we're recording at my home setup, I don't have any guests visit at any point in the process. More often than not, artists experiment less and play it safe when friends are in the room out of fear of embarrassing themselves in front of someone who doesn't understand the recording process. Sometimes it takes good old fashioned dirty grunt work, sweat, and tears in the studio to get the song done, and not everyone wants their friends to see that. Whenever possible, keep the room limited to people that are busy making the record better.
Speed is your best friend when recording vocals. Artists get tired and uninspired over time, so you want to streamline the process as much as you can. Learning your keyboard shortcuts like the back of your hand is a huge time saver. The time you save not fiddling in menus really adds up, and it minimizes the gaps in between vocal takes, keeping the session flowing cohesively. Make use of templates so you don't have to create new track configurations on the fly, and start to develop your own workflow. If you're an up and coming engineer, this is the time where you offer those cheap or free sessions, so you can start to develop the muscle memory and speed that'll help you when the big vocal projects pop up.
You can't physically go in and record the verse for them, but always try to thing of ways that you can improve the experience from the artist's perspective. It'll lead to better records, and more clients.