"The tension between Mel and Robyn was obvious. Even during family dinners, they wouldn't speak sometimes. Robyn would pretend that everything was perfect. No one would ever dare to ask her how she and Mel were doing," the source tells PEOPLE.
The problems seemed to become evident three years ago. "During the summer of 2005 the whole family had a great time hanging out at the Gibsons' Malibu beach house, but at the beginning of , Mel started having severe depression and went back to drinking massive amounts of alcohol. It was all very stressful for his wife," says the source.
"At the same time, Mel was under tremendous pressure while filming [his epic Central American drama] Apocalypto in Mexico. The movie got delayed because of terrible rainstorms, and Mel and his crew stayed in the Mexican jungle for weeks during bad conditions. Mel's drinking got out of control because it was very difficult for him to deal with the pressure of producing the movie."
The source adds: "Mel's family had no control over his drinking and were instead afraid of his bad, unpredictable temper." At that time, "Mel rarely left the house [on location] for other things than business meetings."
In papers filed this week, Mel lists the date of the couple's marital separation as Aug. 26, 2006 – less than a month after his profanity-laced DUI arrest. (Robyn has not specified when the couple separated.)
'Complete Opposites'In terms of their personalities, "Mel and Robyn had always been complete opposites," says the source. "Robyn doesn't drink and wants to have a 'dry' home for their kids, while Mel likes to goof around. Robyn has always been very strict, rigid and at many times very intimidating."
As parents, the Gibsons, both 53, were strict with their seven children: sons Thomas, 9, Milo, 19, Louis, 21, William, 24, twins Edward and Christian, 26, and daughter Hannah, 28. "Their kids are not in any way spoiled Malibu brats. All the kids had to work if they wanted money," says the source.
"While Mel, in the past, spent most of his time working, Robyn always kept their kids on a schedule and was always in charge of everything. She did an incredible job, but as an outsider it's easy to feel that she could be a bit warmer and less rigid when it comes to the kids," the source also observes.