Since we didn't get one yesterday, I'll offer three today: mercurial, jovial, and saturnine
mercurial: adj. liable to sudden unpredictable change; "erratic behavior"; "fickle weather"; "mercurial twists of temperament"; "a quicksilver character, cool and willful at one moment, utterly fragile the next"
jovial: adj. gay; merry; joyous; jolly; mirth-inspiring; hilarious; characterized by mirth or jollity.
saturnine: adj. heavy; grave; gloomy; dull; showing a brooding ill humor; "a dark scowl"; a morose and unsociable manner.
Martindale's behavior was mercurial: he could be grandly jovial one minute and profoundly saturnine the next.