"Covered with" vs. "covered in" vs. "covered by"

I want to find out the differences in meaning among covered by, covered in, and covered with. For example, what is the difference between:

covered with blood
covered in blood

or the differences in meaning among

the mountain was covered with snow
the mountain was covered in snow
the mountain was covered by snow

The meanings are very similar, and these three prepositions can be used almost interchangeably, particularly in the context of your "The mountain is covered with/in/by snow" example. But some subtle nuances may apply.

When referring to a substance that sticks to another, use in or with, but not by:

• The actress was covered in blood, or
• The actress was covered with blood, but not
• The actress was covered by blood.

Another example:

• The ribs were covered with sauce, or
• The ribs were covered in sauce, but not
• The ribs were covered by sauce.

When referring something that physically protects something else, use with or by, but not in:

• The field was covered with a tarp, or
• The field was covered by a tarp, but not
• The field was covered in a tarp.

Use covered with to indicate an unusual amount of something on top of something else; use covered by to connote a covering so dense that the object being covered is completely obscured from view:

• The mountain was covered with fog.
• The mountain was covered by fog.

Another example:

• Our grass was covered with butterflies.
• Our grass was covered by butterflies.

Somehow, the latter (covered by) paints a picture where the butterflies are so close together that I can hardly any the grass at all, but in the former (covered with), I picture a lot of butterflies, just not necessarily so many that I can't see the grass.

When talking about metaphorical coverage, use covered by:

• The roof damage was covered by insurance, but not
• The roof damage was covered with insurance, or
• The roof damage was covered in insurance.

Another example:

• The city council meeting was covered by the news station, but not
• The city council meeting was covered with the news station, or
• The city council meeting was covered in the news station.

Other guidelines are likely to apply as well. This is not a question with an easy and straightforward answer.

All three are in use, and the differences are subtle; but they do exist.

"Covered by" generally means that the covering actually hides the thing that is covered: it would usually be an object (a sheet, a lid, a curtain) rather than a substance, that is doing the covering. "Covered by blood" is unlikely, and "covered by snow" would imply that the snow is so deep that you can't see what it is covering. "Covered by blankets" is a more likely example.

"Covered in" is more metaphorical: the covering is widely distributed over the object, but does not actually hide it. "Covered in blood" is a typical expression, and does not usually mean that every single point on the surface has blood, rather that blood is widespread over it.

"Covered with" is somewhere in the middle, and can mean the same as either of the other two.