Roasting is a process that, by its very nature, is taking away elements that are naturally occurring in the bean; the longer you roast the beans, the more of the bean is lost. Because of this, many of us try to keep what's special and unique about a certain coffee in the cup. Where that becomes a challenge is when a coffee has off-flavors. And off-flavors in coffee are a reality. We've all drank a light-roasted coffee where one of the major flavors contributing to the cup is an off-flavor. It needs to be roasted differently.
The good thing is that there is more than one way to roast a bean. One of the ways that I'll go about it is to first roast the coffee with a common profile; one I am comfortable with, that I know what to expect. And I let the beans sit for a day or so then brew them. I see what the acidity is like, what the sweetness is like. How does the body feel? What about the aftertaste? I take note not just of the great flavors, but also root out flavors that I don’t like.
Then I shape the profile according to the flavors. The key here is to not simply roast to bring flavors out, but to also take flavors out. What I have going through my mind at this point are all things that I can effect in the roasting process: the development of the acidity, the complexity of flavors, the weight of the body, the sweetness and the aftertaste.
Let's say you have a coffee with a really harsh acidity; it's strong and forward, but generic and it overwhelms the cup when you roast it with a common light roast profile. I would try to remove the harshness by developing the acidity further. You can still roast the coffee for the same time and possibly the same temp, but stretch out the Browning Phase. Get into the Browning Phase earlier, adding 30sec to the Phase can have a large effect. If your previous roast was in the Browning Phase from 6min-9-min, get into Browning by 5:30min and let it continue until 9min like last time. By doing that you are giving more time for the chemical reactions to occur, allowing the acidity to develop further. It will take the undesirable edge off the acidity, while making it more complex, hopefully balancing it with the rest of the flavors.
What I’m trying to get at is that a major part of roasting is knowing how to remove flavors you don't want. You can't make a coffee something it's not, but you can make it not something it is. Knowing when to apply heat, or change airflow, when to stretch a phase out, or tighten it up is all a part of it. Knowing your machine, and knowing the tools you have at your disposal is what you need to roast great coffee.