The pair gleefully rocked up to early press interviews hand-in-hand and waxed lyrical about one another.
An HBO source confirmed that during filming the duo were 'inseparable on the set.'
'The two of them were always joking around and having fun together.'
However over time this friendship has eroded. Once the show went to air in February, the pair’s relationship started to become competitive.
A source told LIFE & STYLE that things turned from that point onwards.
'The success of Big Little Lies has gone to both of their heads and there’s now a power struggle between the two stars,' the source revealed.
'Behind closed doors, their egos are clashing and they bicker over the smallest thing.'
The public were given a glimpse into the ongoing feud at the Emmy Awards in September when Big Little Lies won the Outstanding Limited Series category.
On stage, Reese grabbed the statue and refused to let it go – even when Nicole tried to have a turn holding it.
Tensions were further fuelled after Nicole beat out Reese for the Emmy award in the category of Best Actress In A Limited Series.
'There seemed to be some tension between Nicole and Reese at the Emmys afterparty,’ says the Insider.
'There were some icy stares back and forth, kind of checking who was getting most of the attention, which was clearly Nicole.'
For now the stars are continuing to play nice for the cameras when fulfilling their publicity duties. But the insider says this is all just for show.
'The bottom line is that Nicole and Reese really don’t like each other.'