How to become more spontaneous?

Or, how to get out of my comfort zone? In general, I’m not afraid to try new things, and I do, but usually still within my general realm of knowledge. But for some new things I’ll have that voice in my head that says “go for it” but it’s hard to take that step. Any ideas for how take the plunge?

This is not in reference to anything particular at the moment, just a general thing I’ve noticed in the past few years.