That guy looks like David Janssen

At 1:40:00~ There is a scene where the top inter-nation bigwigs are discussing what do since they have been given 24 hours to get out. Niven and Heston discuss killing the empress. To Heston's left, there is a man who has no lines who looks a lot like David Janssen of "The Fugitive" fame.

It doesn't make sense that Janssen would be an extra, as he was already an established star in his own right in 1963.

Still, that guy could be his twin.