LOS ANGELES—Tom Selleck spent most of his late 20s and early 30s dealing with failure. There were failed auditions and failed pilots and failed performances in failed movies. (He played a stud named Stud in the 1970s flop Myra Breckinridge.) All of that ended in 1980, when Magnum P.I. helped him become television’s biggest action star.

Thirty years later, at 65, he’s still working in the action genre. He has a police series, Blue Bloods, coming up in September, and he produces and stars in the Jesse Stone series of made-for-TV movies.

He also has a pivotal role in an action comedy called Killers, which opens next Friday (June 4). Here, he plays a protective father concerned about the relationship between his daughter (Katherine Heigl) and an enigmatic man (Ashton Kutcher) who is, it turns out, a hired killer.

In an L.A. hotel room, Selleck says that if there is a secret to his success, it may be that he has never backed down from an intimidating challenge. “My passion is always there because I get scared,” he says. “Every good part should scare you a little bit. You might say, ”˜I know how to do that and that, but I have never done that.’ Then that day comes and you either overcome it or you don’t, and you either succeed or fail. I said no to Magnum because it was very James Bond-ian and he was perfect. I said, ”˜I want to play a flawed guy.’ If you really watch Magnum I am 6’ 4’’ and he is very physically capable and he is a bit of an action hero, but he whines a lot of the time. He owes people money and he is very flawed, and challenges like that are where my appetites are.”

Although Magnum P.I. still plays all over the world and the Jesse Stone movies do very well in the ratings, Selleck may be best-known to a younger generation for a regular role in a comedy series that’s on TV virtually every day. He says that he was advised to stay away from playing the older lover of Monica (Courteney Cox) on the long-running, now-syndicated comedy Friends.

“It was a risk. Everyone I knew in the business said, ”˜Don’t do that. You can’t guest on someone else’s show. That looks like you are crawling back to television when you are doing features now.’ I said, ”˜Why? It’s a good show. I haven’t done a sitcom since [an episode of] Taxi, and it scares me and I like the challenge.’ I knew it was risky but you have to expand your parameters, because audiences accept me for a lot of things now, but that took time. I think if an audience ever feels you are taking them for granted they might give you one more try, but you should never think you can just show up and they will watch. They’re going to turn on you, and you will never get them back.”