What is the difference between "fill" and "fill in"?

I am confused by fill and fill in. I checked online, and both forms are used in

• fill a hole

• fill in a hole

So I am wondering is there any difference in meaning between them? If not, what's the meaning of in? What is the function of in "fill in"?

In this context, to me, fill in suggests that the hole is removed, i.e. filled with the substance of its surroundings. Fill could mean this too, but is a little less particular. Other than that they seem about the same.

For example, "The man was so big that he completely filled the hole" expresses that the hole was occupied by a large man. But you wouldn't say the man "filled in" the hole.

There are some differences in wider usage. 'Fill' can be something that is designed as a container as well as a hole, whereas 'fill in' implies that it will be filled once and then left that way. 'Fill in' can also be used to mean to inform or to complete.

edit: and in fact it is this meaning of completion that is reflected in only being filled once.

Examples:

I fill my cup.

Makes sense, whereas

I fill in my cup.

doesn't make sense.

Other uses of 'fill in'

I filled in the rest of the detail on the canvas.

I'll fill you in later.