William is a tremendously talented actor who many of my generation first saw (and adored!) in Aliens as Lieutenant Gorman. Alongside this role he has a vast list of other credits which span a huge variety of characters. This is what made William, in my mind, the ideal choice for Walter; a character that both Guy and the audience should find very hard to read and define his motives.
Walter therefore requires an actor of great range and ability, moreover because he is never on screen - his voice is the character. So it is excellent that we have William on board and I very much look forward to see what he brings to the character.