It's scary doing something without thinking, but I reckon everyone needs to have that skill under their belt. This means not weighing it up and not considering the consequences, you just have to do it. If we don't know how to do this then I think we're kind of trapped, because there are many situations in which logic has no jurisdiction yet need to be responded to with action. In these cases doing something is better than thinking about it and doing nothing.
But I think most of all spontaneity needs to happen now, if you've thought about it then it's too late. Just do it; I think one can become more skilled at this, get used to its flavour as it were. There's no technique I don't think, that would mean a deliberating mind would have to be involved which takes time. Maybe it's not something that a person does, maybe you could say that spontaneity is happening all the time, existence doesn't wait to happen, it's doing so much stuff and it's doing it right now without hesitation. So being spontaneous, acting and doing are just taking part in that -- rather than creating it themselves.
And even more importantly, once the act has been committed, life will never be the same again, we have literally changed the world. This is where the scary bit comes in, we stop. Start wondering, what if, what if? Well that can be okay sometimes, but only as long as we know that we have that spontaneity-skill if needs be -- to just do it, to drop the thoughts, bypass the whole brain thing and alter the universe.