The Circle door knocks explained
 

https://www.digitalspy.com/tv/realit...-players-meet/

Paddy has said that they all get knocks on the door at the same time which are fake knocks to create the reactions from them, but when the door knocks they are not allowed to move and must wait in the living room to see if anyone comes in

I thought it might be something like that, unless the producers said "we want shots of everyone looking scared, fake a reaction plzkaythks"

Alex who won last year said paranoia sets in as well & how any sounds in the corridors make them jump & react.

But I figured it be knocks on all their doors.