In terms of Siddig's performance in "The Passenger" I believe it was explained best in this interview
"I'd only been on the set a couple of weeks before we started filming The Passenger
. That episode was my nemesis and a total nightmare that I had to pull out of a hat. I literally got the script twenty-four hours before we started and then received a phone call from the producers twelve hours later saying, 'Sid, we'd like you to portray a different character for this possessed guy you're going to play. We think the best way to portray a different character is to drop your voice an octave. Can you do that?' "I don't think they understood how incredibly difficult that would be. They must have read somewhere that Laurence Olivier had dropped his voice an octave for Macbeth,
" he laughs, "and thought all actors can do that. I couldn't, of course, and by the time we actually started shooting I ended up growling like some sort of desperate tramp. We ended up having to reloop [dub] the whole thing in postproduction."