Yes, overdo any macronutrient (protein, carb, or fat), and the calories get stored as fat. However, with protein 25% of the ingested calories are "wasted" in the digestion process. With carbs, it's 10%, with fat it's 2%, I believe. So if you eat 100 calories of protein, 75 will be used as energy or stored as fat, whereas if you eat 100 calories of fat, 98 will need to be burned or stored. However, this does not give you permission to eat 200 calories of protein in place of every 100 calories of fat. I know this is oversimplified because we need protein for rebuilding tissues, etc, but that is the general idea.
I assume how much is appropriate per sitting depends on your individual body's needs and the type of diet you are on. If you have 400 calories to spend on a meal, there is no harm in eating 100g of protein and nothing else for the 400 calories. Even though only part of it will actually be used for rebuilding purposes, the rest will be converted to glucose for energy. If 400 calories is the right amount of calories for the meal, you won't get fat.