Basically, each suspect has up to four clues that they can be connected to--specifically connected in a way that will lead to the murder being solved. When that connection is made, you put an answer token between the suspect & clue. You're not connecting suspect and suspect, so any number of other characters can show up and interact in these scenes.
Keep in mind that you get to choose how these answers are generated, so can set a scene with anyone in it as long as it leads to a convincing connection between the chosen suspect and clue. The suspect in question doesn't even need to be in the new scene! You touched on that with the last part of your comment; it can be a lot of fun to pull relevance out of what looks like obscurity.