Tā shēngxìng chénmò-guǎyán, hěn shǎo zhǔdòng kāikǒu.
He is taciturn by nature and rarely speaks up.
Tips
memory
沉 (sink/deep) + 默 (silent) + 寡 (few/scarce) + 言 (words/speech). Picture someone sunken deep in silence with scarce words - a still pond, nothing rising to the surface.