Yes, Dialogue should be like a conversation. It needs pauses and punctuation, so in each time the speaker changes you start a new line and indent.
Yes, any time you change subjects you change paragraphs.
Yes, you indent each new paragraph.
Speak to each other
They talk to each other.
In a film, dialog is the collection of spoken words that the actors say to each other.
speak with each other
commas
commas
The north and the south hemisphere are separated by the equator!
nothing
You are two people who are married to each other who are separated.
Genes on the same chromosomes are separated from each other by crossing over. It is one of the final stages of genetic recombination.
Drainage basins are separated from each other by an area of higher ground called a divide.