Audience
Starter:
What if…
There was an alien invasion?
I would probably go all x-com on i and join the fight against them.
An asteroid heading for earth?
Remin the government that there are ways with dealing with that, like sending a ship out to send it out of course.
A time travel machine was invented?
Go back to the past and "fix" my past errors.
Mainstream audience - The genre which appeals to the general public, no actual restrictions like age gender or interests.
Niche audience - Genres which appeal to a certain gender, age or particular interests. fans in this bracket are sometimes known to go to conventions, dressed as the characters from there favourite film or TV series. this is called cosplay and it is not the only science fiction genre which attracts the participants.
Niche movie example
You can tell that this is targeted at a niche audience due to the lack of detail in the poster. it wouldn't have the funding that mainstream movies have as it would not have a huge audience. however, it still has the potential to be a good fiction movie.
Mainstream movie example

Cosplay is becoming a very mainstream thing now. It is a lot more mainstream in America then it is in the UK, but it is taking off. The act of dressing up as a character is completely rad and shouldn't be put down as much as it is.