I think it solely depends on the field or industry the startup is based in. In most cases yes, a lot of people start to work on their projects which eventually turns into a startup while still working their jobs every day. I think it is very possible.
Exactly! It's definitely going to be dependent on the kind of job that you're doing and how important you are to the company. When you find yourself in a position where it's not possible for you to have spare time to do other things, it's going to be very difficult for you to run a new start up and combine it with your job.