- fill something in / something out
Transitive, Separable
Meaning: to write information in blanks
Example: Please fill in/out the form with a pen.
fill someone in
Transitive, Inseparable
Meaning: to update someone with the latest information
Example: Can you fill me in about what we discussed in the meeting?
Craig David can tell you more about this one: http://www.youtube.com/watch?v=dG1F89DetCM