Do I believe in what I write, or say? Sometimes yes, sometimes no. An actor (and for me a professor is in many ways an actor – I would fall asleep on my job if I felt otherwise) should present his lines in an interesting way; he does not need to believe them. It is up to the audience to decide about truth, plausibility, or whatever else they think is important.
"Not a Philosopher", in: Falling in Love with Wisdom: American Philosophers Talk about their Calling, D.D. Karnos and R.G. Shoemaker (eds.) Oxford University Press, 1993, p. 16.