Inside the head of a mastering engineer!
When a finished mix is ready for mastering and gets sent to a mastering engineer, one thing is expected by the artist. The artist expects that if there is any faults in the final mix that the mastering engineer will catch these faults and either correct them or advise on what to do. I'm going to talk about my approach to mastering an artist piece of work...
First and foremost performance means nothing to me and the performance of the musicians has no bearing on my technical decision making. I am not to judge the song or the skill level of the musicians, but to judge the final product from a technical stand point in regards to playback ability and balance throughout the audio spectrum.
Before any processing I'm listening to the final mix for critical errors such as pops, clicks, hum, crackle, noise. Pops and click can come from plosives and "esses" due to improper recording of vocals in particular but can also come from any instrument that moves a lot of air. I listen for hum and noise specifically at the start and tail end of the track. Most of these issues can be fixed in mastering however there can be side effects to the rest of the mix as a result. If there are critical errors i will address these before going further down the processing chain.
Next i listen for imbalance with regard to the instruments in the mix as well as imbalance in the frequency spectrum. Is the kick drum overpowering the mix? Are the vocals too quiet? Are the guitars fatiguing to listen to after a short period of time? Is the center image "cloudy"? Is the stereo image weak? These are all questions that run through my head in an instant without really thinking about them. Again these are just questions, this does not mean that there are always these problems. In a perfect world the final mix would only need limiting and then send it off to the client but mastering engineers are given the job of quality control and thus have to make informed decisions so as not to ruin a mix.
After a couple listening passes to get my head wrapped around the track i will immediately apply limiting to hear how the final mix reacts to a limiter at the standard lufs for the genre I'm working with. The mix may crush itself due to and eq imbalance and sound horrible and at this point i may make some adjustments. and proceed with more processing.
The biggest point is that the mastering engineer is to have an unbiased listen to your mix and be brutally honest and make corrections where necessary. Mastering engineers are a necessary part of recording and releasing a song to the world to listen to. Our job is to listen, advise and repair when nessesary...