【Serious】 , 【grave】 ,  【solemn】 ,  【somber】 ,  【sedate】 ,  【staid】 ,  【sober】 ,  【earnest】  may be applied to persons, their looks, or their acts with the meaning not light or frivolous but actually or seemingly weighed down by deep thought, heavy cares, or purposive or important work.

【Serious】 implies absorption in work rather than in play, or concern for what matters rather than for what merely amuses.

【Grave】 implies both seriousness and dignity but it usually implies also an expression or attitude that reflects the pressure of weighty interests or responsibilities.

【Grave】  is more likely than  【serious】 to be used when a mere appearance is to be implied and it may be used of things with qualities suggestive of human gravity.

【Solemn】  usually heightens the suggestion of impressiveness or awesomeness often implicit in  【grave】 .

【Somber】 applies to a melancholy or depressing gravity, completely lacking in color, light, or cheer.

【Sedate】 implies composure and decorous seriousness in character or speech and often a conscious avoidance of lightness or frivolity.

【Staid】  implies a settled sedateness, often a prim self-restraint, and an even stronger negation of volatility or frivolity than  【sedate】 .

【Sober】 sometimes stresses seriousness of purpose, but it more often suggests gravity that proceeds from control over or subdual of one’s emotions or passions.

【Earnest】 implies seriousness of purpose as well as sincerity and, often, zealousness and enthusiasm.