Si is the bed I sink into when my dominant Fe becomes too much. It's like my little shell when the world overwhelms me. It's where I can go and delve deep into my memories, revel in the comfort of my mind and what's familiar.
Si is also like my guide. It's my experience, it's what I turn to when I'm unsure, it's the standard to which I hold everything.
Si is really internal for me. I'm an extrovert, so Fe-Ne is what the world sees. So when my Si comes out, it surprises everyone.
I seem actually incredibly fun-loving and spontaneous...definitely not how one would immediately picture an SJ. I'm the one who wants to go on adventures and try a bunch of new shit. But deep down, I'm in control of all that. I only like spontaneity when I call all the shots, and when I can return at any time to my little spot of familiarity. I'm terrified of change and I secretly manipulate my whole world to try to keep everything solid. Deep down I need a lot of structure, and I my mind actually has a surprising amount of organization and categories. That's another thing--I categorize everything. Everything in my mind has a label and a place in my mental web.
Also this is a stereotype but I still have a hella good memory and love traditions of many kinds. I'm your girl for remembering exact dates for things and random trivia that everyone else forgot.
Basically, my Si is what makes me an SJ deep down. On the surface I'm your lovable, witty Fe-Ne basket-case, but underneath I truly am the change-averse, tradition-loving, pragmatic, comfy SJ that you'll always read about.
I think it may be very different for ESFJs as compared to other SJs (at least in my experience; I'm not trying to be a snowflake or anything). ESFJs are extroverts and use their Fe-Ne, which often looks startingly ENFP on the outside. But any other SJ can jump in if I'm way off base.