It will take a moment for even the most die-hard fan of ABC’s “Lost” to recognize the scary-thin, massively ripped and totally insane serial killer in “Alex Cross.” But when it kicks in, the realization hits harder than the mixed martial arts blow that the crazed hit man called Picasso delivers in a brutal early scene of the thriller, opening Friday.

The baddie is Matthew Fox, who played noble Dr. Jack Shephard on “Lost” for six seasons. “There are people that are going to be horrified by this,” Fox, 46, says with a chuckle. “They are so attached to the idea of that (“Lost”) character and his ultimate goodness that to show up and see me doing this, well, they might not be happy.” Neither is Tyler Perry, who plays homicide detective/ psychologist Alex Cross in his first dramatic role-a film reboot of the best-selling novels by James Patterson. Perry finds a formidable on-screen opponent in Fox, who dug deep to find darkness. Fox guesses he lost close to 40 pounds from his 200-pound frame during a fivemonth physical regimen before filming, incorporating weight loss as well as body sculpting. “There were no French fries, man,” Fox says, moments after placing two fries in his mouth during a recent lunch at the Four Seasons Hotel. He’s not as thin today as he appears on-screen, and he enjoys the odd fry again, but he still focuses on his salad and healthy club sandwich. “Some of the meals were very, very leafy,” he says. “I was craving all the things I love-pasta, steak, my mother’s pie.” Fox worked on mixed martial arts training along with the intense workouts.

“I played college football (at Columbia). I thought I knew my way around exercise,” Fox says with a sigh. “Either I was wrong, or things have changed drastically. Maybe it’s both. This was full-bore. Many times I felt like throwing up.” For the final step in his transformation, Fox shaved his hair “pretty much to the bone” to present himself to director Rob Cohen. As for the spooky and extensive tattoos seen on-screen? Those were already inked in. “I’ve been getting tattoos for a lot of years, and like a lot of people into it, it keeps building on itself,” says Fox. “I’m probably really pushing the limit of an actor with tattoos.” Cohen says Fox wouldn’t allow pictures to preview his initial visit. “He wanted to surprise me. When I saw him and how ripped he had gotten, I thought, ‘This is no Dr Jack.’ We were going where he hadn’t gone before.”

That included a mental readjustment as well. Fox and Perry even kept their relationship chilly to ensure their tense performances. “We didn’t really interact much,” says Fox. “Those two guys essentially want to kill each other (on-screen). We didn’t have moments where he was like, ‘Dude, you look like a freak.’” “There was no joking around, no relaxing between them,” says Cohen, adding that Fox “stayed by himself on set. He was very intensely internal.” Fox admits that his Picasso character stayed with him even after he stopped shooting each day. “It was kind of incessant. I couldn’t stop thinking about him. I would get back to the hotel room after a scene and do everything possible to take a break, to check out of it.” However, Fox says his mental state had nothing to do with a reallife altercation with a Cleveland bus driver who said Fox assaulted her in August 2011. (Prosecutors did not charge Fox, and a civil lawsuit was later withdrawn.) He concedes that the incident might have been caused by his strange physical appearance. “I know I looked scary, and that was something I underestimated at the time.

You sort of lose track. After five months of progression of changing my appearance, I never said, ‘You have to be careful because you look like a Nazi skinhead.’ “I can tell you this,” Fox adds. “I have never hit a woman in my life. Never have and never will. That’s not who I am.” Though former “Lost” co-star Dominic Monaghan kept the controversy alive with a tweet disputing that assertion, Fox says he’s moving on personally. He will, however, continue to explore the dark side onscreen, playing a Navy SEAL during a zombie apocalypse alongside Brad Pitt in “World War Z,” out next summer. “What has happened in the world has sort of fractured him. He doesn’t know where he fits anymore,” says Fox of his “Z” character. “He’s a definitely a little unhinged.” Cohen believes this dark side will truly highlight Fox’s acting potential. “‘Lost’ shows a Matthew Fox he can easily access. But going to these other places, this is where he can really shine over time.” As for Fox, he just wants the great parts that are coming his way. “If there are ‘Lost’ fans out there expecting to see any version of Jack Shephard ever again _ that was that. My choices going forward are going to be different. Whatever is next might be a good guy or a bad guy. Or something in between.”