A stolid person can’t be moved to smile or show much sign of life, in much the same way as something solid, like a giant boulder, is immovable. Both are expressionless.
It's hard to get excited about the word stolid. It refers to emotionless people or things, and it even sounds pretty dull. Your face may be stolid, as you plod through the unemotional history of the word born in the 17th century of little more than Latin words for "foolish." In some definitions, stolid does have more complimentary synonyms, such as "dependable" or "calm," but these can be overshadowed by other words for stolid — "empty," "blank," and "vacant," to name a few.