Most fiat is principled, you just need to find out what the principle is.
Four somewhat common principles:
Ask what would be there, if indeterminate then decide yes and think about what clues it could offer.
Ask what would be there, if indeterminate then nothing that actively harms or helps, so nothing.
Make something be there that hits a flag (the cameras reveal it was one of the characters lovers), unless this overloads the pacing.
Have something be there but it's another clue that leads to the next clue in the sequence (roads to Rome).