"You pick up on people very clearly I think very quickly. And if you choose really good characters and you give them situations that are funny, provide consequences, maybe provide a bit of conflict, you don't really need to do much," she said.
"When I've done my job in pre-production really well I don't really need to do anything [during filming]."
After they had selected the right people it was all down to set conditions and food, Marion explained.
"In the Big Brother house … we design the house around what we want to happen," she said.
"So if we want a year where there are lots of arguey, conflicty content we will bring the ceilings down, we will turn the lights up … lots of square edges.
"When we want it to be a happy house we have lots of soft corners, soft furnishings, we get furniture in."
Planning extended to what meals were served, with Marion explaining how in one season of Big Brother UK a "rich house, poor house" theme saw some contestants served deliberately bad food.
"We gave the rich house things like doughnuts and curries and food that smells really nice and makes your mouth water," she said.
"[For the poor house] we got someone in from I think it was the army and we said can you design a food plan that has no taste, is really miserable and makes you unhappy, makes you windy … so all you want to to is you just feel like pooing or farting all the time, which makes you uncomfortable."
Tim, who won Big Brother in 2013, said Marion's revelations were "blowing my mind" and confirmed suspicions he had while filming.
"These are all things I've thought I've been a part of," he said. "That process which has rich house, poor house. I've had that diet where I've thought someone has constructed this to make me feel a certain way and I felt manipulated by it."