Jesse Metcalfe (left) and Patrick Duffy, on Dallas season finale, with Julie Gonzalo (inset)
By Melody Chiu and Alison Schwartz
08/10/2012 at 11:00 AM EDT
The title for Wednesday's Dallas finale easily could have been a spoiler within itself: "Revelations."
The label was also appropriate, given that the episode delivered the biggest bombshell yet to detonate on the TNT show – that Rebecca Barnes Ewing is actually Pamela Rebecca Barnes, as in the daughter of oil tycoon J.R. Ewing's original rival, Cliff Barnes.
It was a revelation Julie Gonzalo had to keep to herself when she first signed on to play Christopher Ewing's wife in the oil and cattle-ranching primetime soap opera that is a reboot of the immensely popular 1978-91 CBS series.
"Obviously the crew knew," the actress, 30, tells PEOPLE. "It was kind of amazing when everybody got the final script, and I'm like, 'Ha ha, I knew that!' I'm very good at keeping secrets sometimes."
Still, it wasn't necessarily an easy secret to keep, and Gonzalo admits she might have let costar Jordana Brewster, who plays Elena Ramos, in on her character's alter ego.
"There were moments, 'Oh, where is this going? Where is this character going?' " she tells PEOPLE. "I'm like, 'I know where I'm going – I just don't know how!"
She adds: "We were so invested in every script, and I myself as a person do live day by day and moment by moment, and I don't get ahead of myself too much. … There was so much stuff going on in each script that I'm like, 'I don't even have to.' "
Gonzalo will be back on the Dallas set when production on season two begins in September, and already she is examining her character.
"It just dawned on me that this show is about a family, but it's always about making somebody proud. I'm trying to make my father proud, but I just couldn't show it," she says.
Next up: "There's a lot more back-story next year."