I do illustrations as a side job. The clients and freelancing gigs vary quite a bit. Usually anything from advertising to more traditional media entities like tv-channels. Gaming startups are also common enough.
Usually it's just doing illustrations with what ever technique is the best suited for the purpose. In my case this can be anything from traditional watercolours to a digital painting.
With new clients, the gig starts from a creative brief and a set of sketches to communicate what the finished product will be like in terms of composition, style and color scheme.
The level of detail in the finished piece is highly dependant on the available budget and chosen style.
Getting paid to do something I love. Doesn't really feel like a job. :-) Besides that, it's the creative challenge that each new brief introduces what makes the job really interesting.
Long days spent sitting in front of a computer since I do work mostly in the evenings and weekends outside of my day job's working hours. Difficult clients tends to be cliché with everything freelancing related, luckily I have not had this experience yet. Granted, I tend to work only by referral so that has a huge effect on my clientele.
I've learnt everything software related by myself. Traditional art skills are the most important by far.
The importance of human interaction skills can not be stressed enough. The better the communication is, less complications will emerge. Everybody will be happier.