Ask HN: what to ask for as technical cofounder?

I got approached to help with development for an idea by a couple of non-technical people. Ultimately the implementation will have a technical (web application) and an operational component. The technical portion will be my job.

I have a full time job and will be doing this on the side. There is a possibility that I won't like doing this or won't be able to handle the load and quit, but I do want to try it out and will build at least the prototype.

Finally the question is, what kind of an arrangement is common in these situations? What kind of equity should I be getting and what happens if I abandon the project after doing some initial work?