It's protein or carbs or what so that one can gain weight for a movie project but after completing it would also be easier to get back into shape.
There is no food which makes future weight loss any easier than any other.
If you want to gain weight, you need to construct a diet where you have a caloric surplus over any given period of time.
If you want to lose weight, you need to construct a diet where you have a caloric deficit over any given period of time.
This all depends on your calorie usage, bodyweight, and most of all how disciplined you are at following your plan.
My favorite food for gaining weight is brown rice. Whenever I have a period where I want to gain weight and strength, I eat brown rice with pretty much every dinner. With fish, meat, tuna, you name it. Any of these will do.
All food is the same after consumption. The food is broken down into energy and this energy, if it is not used, will be stored as fat for when your body needs energy later.
The amount of energy in food is called calories. So the more calories in the food that you consume the more fat can be made as fat takes about 3500 calories in order to make one pound of fat.
So if you consume more calories than you need you will gain weight in the form of fat.
The amount of energy you need is known as your Basal Metabolic Rate (BMR) there are plenty of BMR calculators available online which you can use. This is the amount of calories that your body will use in order to keep you alive. Any exercise you do, you should add the amount of calories used from that exercise to your BMR. If that total energy out (BMR + exercise for the day) is more than the energy in (the calories that the food you consume contains) then you will gain weight.
The opposite is also true. If your energy out is greater than the energy in than your body will use your fat in order to get the extra energy for that day. Resulting in your fat stores being consumed and expelled from the body, meaning you will lose weight from the form of fat.