Not quite - you fill out a form by filling in your information; on the other hand, the individual boxes can also be filled in.
So it's "fill out" for the whole form; "fill in" for the individual fields and for the information that goes in them.
"Filling out" can also be used in a human-developmental sense; a grandmother might say of her granddaughter whom she hadn't seen in a long time "My, you're filling out nicely, aren't you?" However, when applied to human beings this has a connotation of ripening or sexual maturation, and so it would be extremely creepy in most contexts other than the grandma/grandkid scenario. Edit: as @Jackson Pope reminded me, "filling out" can also mean "gaining weight"; however, I would still be very careful using this to refer to (for instance) a significant other, as it might lead to a night on the couch.
"Filling out nicely" can also be used to refer to team rosters, music festival lineups, etc. - things which start out empty but must be filled to be useful. - Pitchfork Music Festival Filling Out Quite Nicely
One can also "fill in" for another person; that is, take over their duties while they are absent or unavailable: Bundy fills in for LA third-base coach Wallach