Andrew Lincoln is darn good at keeping secrets.

The star of AMC’s powerhouse zombie apocalypse series, “The Walking Dead” — which ends its second season tomorrow night — has been able to keep a bombshell under his sheriff’s hat for nearly a year-and-a-half.

“My parents called me after the [Season 1] finale and asked me what Dr. Edwin Jenner [Noah Emmerich] whispered into my ear before he died as the CDC headquarters blew up,” says Lincoln, who plays the survivors’ leader, Rick Grimes.

“When I didn’t tell them they got pretty upset with me,” he says, laughing. “I didn’t even tell my wife [Gael Anderson].”

Grimes’ deer-in-the-headlights reaction to Jenner’s message raised the hackles of viewers, who promptly bombarded Twitter and spoiler Web sites — and even pored over the show’s comic-book progenitor — looking for answers. Nothing but educated guesses and rumors surfaced — until last week’s episode, when Grimes was forced to stab Shane (Jon Bernthal) in the heart, killing him instantly. Before Grimes had a chance to catch his breath, Shane was already on his feet in flesh-eating zombie mode.

“I knew what [Emmerich] told Grimes would transform the show,” says Lincoln. “Some of the cast members asked me [about it], along with when their characters would be killed off.”

Well, the jig is up, and tomorrow night’s season finale, which is expected to top last week’s stellar viewership (6.8 million), will unveil the dark secret.

The “Walking Dead” episode which aired three weeks ago teased tomorrow night’s bombshell when the group decided to put Dale (Jeffrey DeMunn) to eternal sleep.

The character, who was mauled, but not bitten, didn’t rise up because he was shot in the head — the only way to execute the “walkers.”

“There’s been a lot of character development the first two seasons ,” says Lincoln.

“Things are really going to intensify next season.”