Take 30 minutes and write it down in your Great Ideas document.
Ignore it for a couple weeks.
Hopefully you'll forget about it and get back to executing the great idea you were already executing on.
Ignore it for another week.
If you can't get it out of your head then talk to a few friends who are used to hearing your wacky ideas and will happily vomit on them.
By this time your brain has probably had enough elapsed time to process.
Now go talk to customers, investors, et al.
If you simply go from an idea to talking to customers/investors and then don't follow through, then you start to appear to be indecisive. After a couple times it's "ugh, Lev's got another idea." That's a downside of lean.