I've been in this situation a couple of times - as Alice. What it tended to come down to was my other opportunities were well-paid jobs that I could take but that didn't have the potential pay-off of a startup. Because most startups fail the choice was between high guaranteed income + not much fun or a lower income, more risk and more fun.
The answer in both cases was that Bob produced a sliding scale of salary vs. equity. The more salary I took the less equity I got. If I wanted the startup life without the risk I took a bigger salary and got less equity. The only question then was what the top and bottom ends of the scale looked like.
As the other comments point out, risk is a dynamic and subjective thing, so it makes sense to review the ratio from time to time to make sure it's reflecting reality.
The answer in both cases was that Bob produced a sliding scale of salary vs. equity. The more salary I took the less equity I got. If I wanted the startup life without the risk I took a bigger salary and got less equity. The only question then was what the top and bottom ends of the scale looked like.
As the other comments point out, risk is a dynamic and subjective thing, so it makes sense to review the ratio from time to time to make sure it's reflecting reality.