WordNet sense 1 (having or revealing little emotion or sensibility; not easily aroused or excited):

stolid

WordNet sense 2 (unsusceptible to or destitute of or showing no emotion):

unemotional

WordNet sense 3 (deliberately impassive in manner):

deadpan, expressionless, poker-faced, unexpressive

WordNet sense 4 (not inclined to talk or give information or express opinions):

uncommunicative, incommunicative