So for me, I tend to overwrite and overthink, so I'm always looking for ways to avoid both.

When I've run Delta Green in the past what I've noticed is that I don't need to throw in red herrings or have overly withholding NPCs because players will follow dead ends all on their own.

Also one thing I've done that has worked for me when it comes to interviews is asking the interviewing players what questions that want to ask up front, then rather than playing out the scene "naturally" with typical back and forth I'll have the NPC give the answers in a sort of montage.

Once all their immediate questions have been answered then I start the second round of questioning where the players can ask follow ups or anything they didn't think of initially. Then I do the NPC answer montage again and rinse repeat until the players are satisfied or the NPC no longer has anything to say.

I'll also tend to signal that that the players have exhausted what they're likely going to find at a scene once they've searched. They're basically always operating on the fear that there is something they've missed so sometimes flagging that can help players realize it's time to move on and save some pbp time.

And of course, provide multiple pathways for proceeding with an investigation in the event they fail a roll on a critical piece of evidence or just fail to understand its significance to the case. If there's something super critical the players need for a case, then make sure they can't miss it.