It expressed that one simple overriding fear: What if you can't?

We fought from simple fear, which was our motivating power.

But, for the most part, it was simple fear.

I came to know that it is simple fear.

The helpless feeling he endured now was worse than simple fear.

Maybe it's a simple fear of singing in public.

Maybe it was plain, simple fear catching up with her.

That her words moved me from simple fear into terror?

It was more than the simple fear of death.

Perhaps it was simple fear of what he'd do if he found out.