When it comes to working on established licenses, I'm a bit of a research nut. The more material I can get my hands on and add to my reference pool, the better. When it comes to long-running established characters I can't read everything, but I try to hit key stories and read the latest half-dozen appearances of that character so I know how they're currently being portrayed.
Reading for research is different from reading for fun. When I sit down to read for research I have a pad of paper and pen beside me and, as I'm reading through, I jot down things that jump out at me:
- People, places, names, including supporting characters, nicknames, important locations. Any details that may end up showing up in my stories as well.
- Plot lines, including important changes in the character(s). When they happened and why.
- Questions I have as I read. These usually get answered over the course of the story, but if they don't then maybe that's a plot thread I could use down the road.
- Personality traits. How does this character act? How do other people feel about them? Some characters can vary wildly over the years, so I try to read a bunch of appearances and then make my own decision about what feels the most consistent or feels 'true' to me based on what they've been through.
- Key lines of dialogue. The way a character speaks, verbal tendencies, catch phrases, all that stuff.
A lot of times I'll start by reading a few key issues and those will lead me down a rabbit hole of other reference. That's the nature of complex continuity spread across hundreds of comics in something like the Marvel Universe.
If there's a wiki, I'll bookmark key pages and use that as a way to find which issues I need. The Marvel Wiki has a listing of character appearances you can order by release date and that's been invaluable for me as I track back through a character's recent history.
I try not to rely on plot summaries or overviews on Wikipedia. They can be an okay starting point for research, but they carry the bias of the person who wrote them. Sometimes they omit key details or read quite differently from the source material. If I have the time, I read the original comics so I can make my own decisions about character and plot.