There’s a certain protocol for reporting profiles, whether short or long, or for newspapers, magazines, or online. The tasks below are listed roughly in order of importance, but they are all steps you should take when doing long or short or long profiles. Of course, the longer your article is going to be, the more time you should spend on each step. (To avoid pronoun awkwardness, I’m going to refer to the subject of your hypothetical profile as “she.”)

1. On more than one occasion (if possible), send time observing her in action, doing the thing that makes her interesting or noteworthy. If a scientist, watch her in the lab; if a chef, in the kitchen; if a reporter, out reporting a story. This will not only give you an insight into how she operates, but will hopefully provide a scene or scenes that will lend drama and color to your story. Spend as much time as you can on this and be, in Henry James’s phrase, “one of the people on whom nothing is lost.” The ideal (though admittedly rarely attainable) goal is to make the person absolutely sick of having you hanging around all the time.
2. If applicable, experience and reflect on the thing the person does. If a scholar, read her scholarship. If a painter, look at her paintings. If a chef, eat her food. Et cetera.
3. Read everything that has been written about the person.
4. Interview as many other people as you can about the subject of your profile. These interviews will fall into two categories. First, people who know and/or have worked with her and can provide insight into her work and personality. Second, impartial experts in the field who can assess her contribution and importance. For example, if the person is an architect, you could talk to an architecture professor.
5. (Don’t do this before doing steps 2, 3, and 4, as these will help you come up with good questions.) Sit-down interview or interviews with the person. This is necessary, but it’s not going to be as fresh as 1, because an interview is an artificial situation. So use the interview primarily to get facts, details, and anecdotes, as opposed to quotes. (In your story, use long quotes only if the person is a great storyteller or talker. Otherwise, use short quotes of one or at most two sentences.)