I agree with RK - having known quite a few actors myself, they are simply people and their intelligence varies. Are actors emotionally stable would have a different response from me though
Also, I try not to judge people too much on one interview. Many things can affect how they behave on the day - and being a good interview subject isn't really a valid indication of intelligence, it simply means you give a good interview! One of the most painful interviews I ever saw was with Rowan Atkinson. Despite being a highly intelligent man (try not to judge him on Mr Bean) he came across as a stuttering fool with nary an idea in his head.