Pursue a passion project or side business alongside my full-time job

I have an idea I've been sitting on — a business, creative project, or side venture — and I keep putting it off. I have a stable job and I'm not trying to quit it, but I want to actually build something. The real question is whether I can do both without burning out, and whether this is a genuine priority or just something that sounds good in theory.

Determine if now is the right time to commit real hours to a side project, and what the actual cost looks like — in time, energy, and focus — before I start.