IMO,

INT is used more for locations that the viewers need to see inside such as a building or home or a car. EXT. is used more for locations that the viewers need to see outside such as the street or outside a building or car. it just provdes the readers with a certain perspective on looking at the scene.  

Any technicalities, descriptions can solve. it provides information to the director on what angle he should film from. And gives you space in between these headings so it does not look so rapidly.

If it's tacky and unprofessional why is it a rule to always write a heading starting INT. OR EXT.? It's not. Helps the reader understand where the location is.

Gabe

i always write  INT. CAR   when the scene takes place inside a car - which is correct.

and if it's a convertable car i just write.... INT./EXT. CAR - MOVING which is acceptable and used as an expample in the Screenwriter's Bible

INT. and EXT. are about the only camera directions permitted in a spec script. They are not used to designate where the action occurs, though the action usually does occur there. These are used to indicate where the camera is placed in relation to the location indicated. You can write EXT. HOUSE, but through the action indicate that the action is being seen through the kitchen window and actually inside the house, though viewed from the outside. To cross the line, each indicator shows where the audience is. The action paragraphs are used to tell where the characters are.

INT./EXT. is rare, but completely allowable when the camera will pass in and out of something, usually a car. Again, this won't happen a lot, but it's there.

These directions are a part of the screenwriting process, and their use is required, so it can't be considered excessive.

How would you right the time of day when the enviroment is in space? After all, there is no night or day? Would I just write

EXT. SPACE

Any help would be appreciated!

You don't.  If you don't know what time of day it is, you leave it out.  Examples:

EXT.  DEEP SPACE.

INT.  SPACE SHIP GALABO.


Phil

Thanks Phil.

How would you write a header when the scene takes place underwater?

INT. LAKE- NIGHT

This doesn't seem right? Anyone know the correct answer?

I don't know what the "correct" answer is, but I always just use;


EXT. UNDERWATER

Hope this helps.


Matt.

Thanks Matt. I'll use that.

This is a judgement call.  I'd personally use exterior with underwater scenes, unless it's an indoor pool.


Phil

I thought on top of water is written

EXT. LAKE- DAY

And underwater is written

INT. LAKE- DAY

But then again, I'm the amature.

Another option you have is something called a subheader, Zack.  This has come up before.

Once you have established the scene, not every header needs to have INT. or EXT.

It could read like this:


EXT. LAKE – DAY

A rowboat sits on the lake.

UNDER THE WATER

A giant shark swims towards the boat.

IN THE BOAT

Zack baits his hook, unaware of the bloody fate that awaits him.



Subheaders can free up your format, and help you move the narrative quicker.

But you do need to go back to INT. or EXT. when you begin a new scene.

Thanks Bert. It seems Subheaders would work really good for a scene involving a moving car.

Since there is neither day nor night in space, you actually would not specify either, as Phil stated. The same would apply to inside (or outside) a space ship that isn't in an environment where day or night would matter as well as any slugline for a scene where there is neither day or night such as an enclosed room or non-locational dream states.

It's all about clarity for this one, since you want to be specific as to where the camera is in relation to everything else going on under water. Frankly, I am of the persuasion that if you're underwater, you would be INT. LAKE or INT. OCEAN.

Some might argue, though, that if you were looking at a submarine and then going inside that sub, you'd be INT. OCEAN and then INT. SUB which would be weird, but again, it's going to be about clarity. Technically, if you were writing about a school, you could be INT. SCHOOL *or* EXT. CLASSROOM while standing in a hallway outside a classroom, and in technicality, both would be correct.

That being said, there is no truly correct answer to the question. As long as the way you've written the slugline is correct and it is completely clear where the camera is in relation to everyone else, then it doesn't matter how you write it. But from a vernacular standpoint, when you go swimming, you are *in* the water, which would imply "INT."

As for moving cars, that's a whole other debat topic. Some would say INT./EXT. would be appropriate there at any time, while others would argue that it would depend as to whether the camera ever left the car. Anyway...