I interview a character. If he's not cooperative, I interview a different one. Be careful though, each character will declare he is really the protagonist. It's because they are the protagonists of their own stories. Don't be fooled. (And if you have a secondary character who doesn't try to claim to be the protagonist, he's probably passive or flat).
The thing with interviews is you have to let the character "talk" even if he's trying to tell you the events or plot are different from what you think. Sometimes we get jammed when we treat a character like a plot paperdoll, trying to "make" him do things he isn't motivated to do.
Another thing I do is read something tangentially related to the story. For example, my WIP is set in the early '70s. For inspiration I went to an antique store and picked up some periodicals from the same time frame. There is a huge difference between filtered nostalgia or our generally accepted cultural touchstones, and what stuff was actually like back then. For a contemporary story, do some casual reading of material related to the subject, themes or the main character's interests.
Something else I'm trying that's new to me, but really helpful, is a big, color coded spreadsheet. It's sort of free-flowing with hunks of lists in different shapes: Time periods in the book, the various characters, physical objects, slang terms, stuff I need to research. I've probably not described it well. The point isn't to create something just like it, but create something that reflects what you're trying to write. Kind of like a corkboard with inspirational items tacked to it, it's a non-linear representation of story elements and context.
Good luck getting unstuck!