Over the past year of training people in a variety of settings I've come to think about Motivational Interviewing as one leg of a 3-legged stool that makes up many people's job functions:
Leg 1: Information Gathering
Leg 2: Problem Solving
Leg 3: Motivating
It's key to be able to recognize what kind of conversation you are having and to think about how these conversations sound differently from each other. Information gathering includes asking a lot of questions, and while good information gathering would still contain reflective listening, your work is likely driven by a desire to "fill out this form" or "just get the information" you need.
Problem solving also often includes a lot of gathering information, but its hallmark is that it sounds like providing solutions. While there are good (Elicit- Provide - Elicit) and not so good ways (lecturing) of giving information, it still usually doesn't sound like motivating.
Finally, there are motivating conversations. In an MI style, these conversations will collaboratively set an agenda and elicit the reasons why one would consider changing or working toward the target behavior or activity.
So how do you know when it's time to motivate or when it's time to problem solve? My easy answer is that when you feel stuck with a client or you've had this conversation before and there hasn't been any movement toward change or action, then motivation likely needs to be addressed. Some tips in choosing the right "leg:"
1) Identify when it's time to use MI,
2) find that target behavior and negotiate the agenda, and
3) ask about the good things that would come if they decided to make a change.
Now you're on your way to a "Motivational" Interview.