Often it is said that SJs are very serious on the job, but that they can turn it off at home, as if by a "switch." That didn't seem to make sense to me at all, but when those times came up in real life, I looked at them and realized I do have a serious mode and a more relaxed mode (but not really the opposite of serious). So I think they were right.
I do that, Cimarron. A lot. Perhaps in terms of the switching part, yes, but not necessarily "serious" to "goofy".
Actually, I have a name for it. I call it my "mask". I work in customer service, (Produce Clerk now, by the way. Day I turned 18 my manager promoted me. No more bagging.)
Anyway, my mask involves.....well, acting like one of those friendly Extroverted people. This mask of mine provides the illusion of friendliness and extroversion. That's the only part of my job that requires a "mask". I am comfortable with the rest of the aspects required, such as servitude, preparation (of food, etc), moving fast and getting things done, etc.
Here's where the switch comes in. When I clock out and hop in my car, I almost instantly feel the impact of all the energy loss. Physical energy loss (to a minor degree), just from working, but....my actual energy that makes me feel alive, (sounds epic, but I don't know how to word it differently.) is almost gone.
I suppose this is just my introversion kicking in. I tend to shift like this, from extreme to extreme. I'm most comfortable and most energized when I can be alone for an extended period of time (minimum of six hours or so, to a day or two.) However, being at work (or anywhere else) drains me. I wish it wasn't like that, but I can't help it. (On the same token, however, I *like* it that way. Odd?)
So, I return home and go to my "cave". (My bedroom. How it acquired that nickname is related to my high introversion, but it's a story for another thread.) Back in the cave, I usually sit and read, play RPGs, write, work on something, or listen to music. Most of my time at home is spent alone, except when Mom insists that I come downstairs.
(She is also an ISTJ, but is less introverted than me. She needs alone time, just like I do, but not nearly as much. I think the amount that I need she considers to be excessive, and hence why sometimes we disagree. But 99% of the time I am in excellent standing with Mom.)
Now that I've totally derailed your perfectly good thread, I suppose an apology is in order. I'll quickly sum up what I should have put in here by itself:
At work = Most of my ISTJ qualities are present, but the "I" is hidden with a very large, bright, friendly "E" mask.
At home = All of my ISTJ qualities are present, and I'm more likely to be openly sarcastic, pessimistic, and withdrawn. (By openly withdrawn, I mean it's clear that I want to be alone. At work this is invisible.)