ceremonial, formal, grave, sincere, in earnest, earnest, sobersided, serious, sedate, humorless, more...

cheerful, jovial, lighthearted, playful, merry

serious, dignified, solemnity, solemnize, solemnisation, solemnization, more...

non, long, other, such, more, silent, how

serious expression,   serious look,   serious tone,   serious face,   overly serious,   too serious,   somber and grave,   grave; seriousness,   self serious,   gravely voice