The controversial broadcaster joined the ITV breakfast show's presenting line-up last year, after impressing bosses with a guest stint, but it seems Susanna wasn't quite as convinced by his hiring.
Speaking on an episode of 'Alan Carr: Chatty Man' that will air on Thursday (28 April) the star recalls: "I literally thought they were joking.
"They told me and I went, 'Ha-ha that is hilarious… Really? Seriously?', and then you came along and it seems to work."
The pair have formed a unique partnership on 'GMB' characterised by Piers' attempts to flirt with Susanna, and her quick knock backs.
As Alan addressed speculation about their relationship, Susanna was swift to shut down speculation.
"It's not an intense sexual chemistry," she says, despite Piers insisting that it is.
When the chat show host asks how drunk she would have to be to sleep with Piers, Susanna jokes: "You haven't got enough alcohol in your stores."
"I have already had enough," Piers laughs.
Susanna then claimed their banter was all in good humour, adding: "Oh, it's all bluff and bluster. He is married to a very beautiful woman."
Catch the pair on 'Chatty Man' on Thursday (28 April) at 10pm on Channel 4.
'Good Morning Britain' airs weekdays from 6am on ITV.