A scene exists for one of three reasons - or a combination of them. If it doesn't have any of these purposes then it can't truly be called a scene.
1) To advance the story.
2) To advance the characters.
3) As a transition - from one strand to the next.
This means that each scene should contain something new... not for the audience, but for the characters themselves. Remember, nothing should be there just for the audience. That's bad storytelling and known as exposition - classic examples are when two characters tell each other things they already know just so that the audience can hear it. (I know it's a weird concept, but the characters don't know there is an audience.)
So - in the first scenario one or more characters will be in a situation where they learn something new, or are faced with a new challenge, or resolve to take some course of action.
In the Second, what happens in the scene will advance the growth of one or more characters - they will be changed in some way.... Whether it is provoked to anger, or violence - or finding courage - or realising they are in love.
In both cases this happens through a combination of action, reaction, and interaction..... and action doesn't mean physical ( though it sometimes is) - it could be someone refusing to speak to someone else.
WHERE SHOULD A SCENE START?
It should start where the drama starts. I know that sounds obvious but you'd be amazed how many badly written scenes DON'T. A lot of scenes are "padded" with answering doors, inviting people for cups of tea etc before the drama REALLY starts. You don't want to bore people so the trick is to skip as much of the padding as you can. That way you have more time for the characters to react to what is really going on.
WHERE SHOULD IT END?
Well obviously where the drama ends... but that's not as easy as it sounds.
Let's say for example that Leah has come round to tell Sally that someone is going to kill her. Well that's the most dramatic bit - so should you end there?
NO - because we need to see Sally's reaction...is she frightened, does she dismiss it as a joke....and then there may be Leah's reaction to Sally's reaction - or it may lead to them both taking NEW action. So once again it goes back to the first rules.... When it stops advancing the story or the characters - the scene is over.
WHERE SHOULD A SCENE BE SET.
Many people look for exotic locations but sometimes that works against a scene... For example, a couple arguing out loud in a public place has to be written differently than if they are in private - so choose the most NATURAL location for your scene... the bedroom and kitchen for example are more intimate places than a lounge room or a hallway. If you put an intimate scene in a public place then more tension will be raised - things might NOT be said which could be said in private. Set the scene in the most logical place for that scene to happen. If it starts in the house and then suddenly they're on the beach...remember that they would probably have been talking in the car ON THE WAY to the beach...just like real life.
WRITING A STAND ALONE SCENE.
If you have to write a stand alone it is wise to either write a paragraph BEFORE the scene starts to tell us what the story is leading up to it - OR - to put a "header" in the big print of the scene itself.
For Example:
"Leah has told Sally that Alf has been secretly in love with her since she was 13 and has a suitcase full of photos of her. Now Alf has come to the house on some pretext - and Sally is afraid. She HAS to find out the truth."
THEN you can start on your dialogue with the set-up already in place .