I usually put some amount in a cup and add a very small amount of water to start mixing. This is usually effective as long as the amount of water is very small compared to chocolate, in which the chocolate still has a strong taste. I thought of using milk, but I do not want the chocolate's taste to be faded (milk has a taste compared to water being neutral ).I also tried using the microwave, but that was pretty useless.
I want something really fast and reliable.
If you're a student like me and forgot to get milk from the grocery and only have a jar of nutella, then do what I do. Just add a bit of water (test with small amounts) and stir. Keep doing that until you get the consistency you want. Still tastes good even if you use water, nutella's flavour is pretty strong to mask the presence of water. Quick and effective.