I know him best from Cul-de-sac and The Exorcist (his last film, apparently).

And I haven't seen either of those! Cul-de-sac has always interested me in that it's consistently discussed in terms of Beckett and Harold Pinter, both of whom I like. Would you recommend?

(Seriously, watch the one-man Beckett. I hadn't thought any of MacGowran's stage work was recorded and I'm really, really glad.)