How to express looking from the outside, and inside, the “outside-look” and “inside-truth”

Question

Many times, things are not what they look like from the outside. What are the two words describing such outside-look and inside-truth?

For example,

He looks casual, but he is a sincere person deep inside.

This is describing a person, about outside-look and inside-truth, I want that two contrasting words that summarize/lead the cases. I meant,

Skin deep, He appears casual, but deep within, he is sincere.

That Skin deep and deep within are the two contrasting words that I'm looking for. What are they? Especially when describing a matter, a situation etc.

superficially and actually? What else? What are the best phrases to replace the following contrasting words that summarize/lead the cases:

From outsider look, this programming language lacks such feature, but what's really true is that it has a much more powerful one.

Answer 1

"Skin deep" and "under the skin" are a reasonably common pair. Here, "skin" is not limited to an organic sense, but can include things like the skin of an airplane or a computer's case. It may not do so well with relatively homogenous things, or where the concealing outer layer is quite thick in relation to what's inside; talking about the amazing crystals hidden within the "skin" of an unopened geode would, I feel, be odd, because rocks don't have skins: they're just rocks all the way through (and the part that looks different is pretty deep inside).

"On the surface" and either "under the surface" (if there's enough space between the uses to keep the similar words from clashing) or "deep down" also work, expressing something a bit more like an ocean, where the top appears fairly barren, but there are millions of fish and microorganisms swimming below.

In either case, it's often more useful to take just one of the pair and allow the other to be implied.

For your specific example, I might take something like the latter pair, like this:

On the surface, this programming language appears to lack such a feature, but if you look deeper, it has a much more powerful one.