and Sam Mendes had a trial separation last year.
The couple - who announced yesterday (15.03.10) that their six-year marriage is over - reportedly split up while working together on 'Revolutionary Road' with Leonardo DiCaprio
last year, but reconciled after realising they were unhappy apart.
A source told the Daily Mirror newspaper: "Working and living together was incredibly intense and basically overwhelmed their relationship.
"The pair have a very different work ethos - Sam wanted to switch off after a long day and Kate liked to analyse every nuance of her performance. It was just too much."
After filming ended, Sam flew out to Florida to work on new movie 'Away We Go' but, despite their troubles, the couple - who raise two children together, son Joe, six, and Kate's daughter Mia, nine, from her marriage to Jim Threapleton - struggled to cope with being apart and pledged to spend more time together.
The source added: "After being in each other's pockets for so long, this, ironically, also proved really difficult.
"So last year they both made a commitment to spend the year together and do less work. But it actually ended up with them going a bit stir crazy.
"They are both really driven and were arguing more than ever before. Kate even did a really small amount of filming to try and ease the pressure and tension that had built up but they had a series of rows over projects this year and that was the straw that broke the camel's back."
Despite only confirming their separation yesterday (15.03.10), rumours have surfaced that there were cracks in their marriage since January, which have been denied.
However, friends of the 34-year-old actress have revealed she and her 44-year-old spouse have been living apart for some time.
One pal said: "They have been living separately for a while. There had been some huge rows behind closed doors before they decided to formally separate."
The couple - who have not been seen together in public since last June - are hoping to keep their split amicable and are keen to avoid a court battle over custody of Joe and the splitting of their assets.
A source said: "Kate is devastated. She thought their marriage was forever."