How do I add a little bit of personality to my SOP without turning it into another "trembling steps" narrative? I wrote my SOP very closely to JeffE's answer here: Choosing research ideas to include in a statement of purpose Just a very straightforward, all research no BS statement.

My 3 letter writers liked it quite a bit and said it was very well written and "ambitious". However, I received a comment saying that, while the technical aspect is very solid, I should "say something more about myself as a person". He said I should elaborate on "what drives me and my interests, and what is it about my research that makes it so rewarding." I asked him for clarification but didn't get a very clear answer so I thought I'd ask some academics what they think.

I'm very passionate about my research (as is expected of anyone applying for a PhD, nothing new here), and after 3 years of research experience in multiple areas of physics, and 4 semesters of TA'ing, there's honestly nothing else I imagine myself doing. It's very simple, I really have no other explanation. Research is rewarding because I enjoy discovering and learning new stuff, period. I don't want to make the world a better place (would that I could), and I haven't been passionate about solid state physics since I was in diapers. I'm not one for cliches.

My introduction currently discusses: a) my general research interests in two sentences b) my goal to work in academia. 

How should I add a flavor of personality, as my professor suggests?
I received similar feedback from a professor who doesn’t subscribe to the “trembling steps” narrative, so I felt it was advice worth following. I navigated the problem by adding a “because...” to a few of my sentences. So you could say something like “I did research on X for [number of years] because I feel it is important/worthwhile/interesting etc.” You’ve prepared yourself for grad school and have lots to show for it, so if you start thinking about why you decided to do those things, it’ll make it easier to add a “because” here or there, which should make your statement stronger. 

Think about your field and what you want to contribute to it. Why is your field worth studying? What makes it important? How are your unique attributes going to advance your field?

I would avoid directly saying something about yourself as a person. The wording of the statement should be enough to convey your voice, and your accomplishments discussed in the letter speak enough to your personality and passion. Just like in creative writing, show don’t tell. Ask yourself what you want the readers to know about your personality, and rephrase your existing sentences in order to convey that sentiment if you feel it is lacking.