The camp TV presenter - winner of series two of the reality show - was in the garden when the tree whispered his mystery challenge to him.
The tree said: "Oi - Trolley Dolly. It's me, the Tree. You're all a bunch of fame-hungry wannabes in my eyes. And you're gonna prove it!"
Brain asked how, to which the tree replied: "How would you like to win them lot some cocktails?
"You've got one hour to get three of them to confess to you, separately, that they think they're the most famous person in this godforsaken house. It's gotta come from them and it's gotta come natural."
Brian first approached Nikki in the smoking area and told her she was the most "memorable" from the seventh series of 'Big Brother'. She said she didn't see it that way, however.
He then spoke to Ulrika, but she too refused to be manipulated into saying she was the most famous.
Annoyed and with time running out, Brian tried to strike a deal with the tree.
He asked it: "You know how you say I'm nice? What about f**k that s**t? What about I get the cocktails and they don't?"
The Tree refused but said it liked Brian's "mean streak".
Meanwhile, the housemates have discovered that Makosi, Ulrika, Nick, Nikki, Chantelle and Nadia will all face the public vote this week.
After successfully passing their majorette task, the group were handed a set of batons with their names on them, which they all had to pick up and read in turn to find out who was nominated.
Makosi also faces another shock today (01.09.10), when it was announced Anthony Hutton - whom she famously romped with in a Jacuzzi during series five of 'Big Brother' - will enter the house as part of a task.