Figments of the imagination should be translucent...
...and should not affect the environment around them.
When the Gerry Butler character walked in the sand, he should not have left footprints. When he went into the water, the water should not have splashed, nor should his clothes have gotten wet.
The best figments or ghosts are translucent, like Cary Grant in "Topper." Having the "imaginary" friend interact with Jody Foster in a real life setting made it look fake, and the effect of an "imaginary" friend was lost.
Of course, "A Beautiful Life" and "The Sixth Sense" didn't go with translucent apparitions, but those movies had excellent surprise endings....