It seems like everyone has a horror story (or 10!) about a salon experience gone terribly wrong. Most of these situations that I hear about seem to come from a breakdown in communication. I feel compelled to share my insight, since I have been both behind the chair and in it.
As a stylist, it is your job to lead the conversation and make sure that 100% understand what the client is trying to tell you. That is a tough job because sometimes the client isn't sure what they are trying to tell you or even what they really want. How do you strip down what they are saying and really get it? Start by asking questions. You may want to ask the same question in a different way a few times to make sure they are giving you a consistent answer. That confirms they understand what you are asking, and you understand what they are saying.
You will be surprised what they think an inch is, when you ask them to hold up their fingers to show you. Show them where the end result of the length would be. If taking off 2 inches will have the end result sit at their collar bone, show them where that is. If they actually wanted it above the shoulders, that would be more length. Save yourself from doing the haircut twice or having them be upset that you can't put the hair back on once it's been cut off.
If this is the first time that you are meeting the client and have never touched their hair before, sit down with them to have a thorough consultation before going to the shampoo bowl. Find out if the texture that you are seeing is the natural texture. Just because it is board straight when they come in, doesn't mean that it is the hair's natural state. If it has been flat ironed, find out if they do that every day or once a week.