I think on the surface, ISTJs are very serious. Most people that know ISTJs probably only know them on somewhat of a superficial level. On this level, all people are basically treated equally, and interactions are fairly limited to only what is necessary (asking questions, getting responses, etc.) I hate having to waste my time on unnecessary conversations or gossip especially if I'm never going to talk to the person again.
In order for me to notice someone, they honestly have to repeatedly be in my life in some way. I take in data about people over time and eventually form opinions and thoughts from what I have observed. If I like what I see, I will either initiate a friendly conversation or the other person will take the initiative. Honestly, I don't think I'm a difficult person to get to know...it's just that I have to have time to get to know you a little first before I become "un-robotic" around you. I definitely one of those people who warms up to ya. Perhaps that is why ISTJs like organizations so much -- it allows us a chance to interact with the same people in a variety of circumstances. I know most of my friends come from class or work -- places where I see them regularly.
So anyway, those that don't really know me (or that I don't really like based on my observations) probably see me as being really business-oriented and serious. My true friends know better. Once I warm up to you I am really laid back and goofy. I joke around and laugh constantly with my real friends. As for being serious during play -- I don't really think I am. I suppose I'm probably more reserved than the average person, but that's just how I am naturally. If you really wanna know what goes on in the mind of an ISTJ, get to know them well, and then get them tipsy. Hilariousness will ensue.