Like apprenticing to knighthood, becoming a Wagnerian heldentenor is a long process, and young singers over-eager to take on a five-hour opera “do so at their peril.” So says John Treleaven, who stars as the tragically smitten hero of LA Opera’s production of Richard Wagner’s “Tristan und Isolde,” which opens this Saturday.

FineArtsLA spoke with the 52-year-old Cornish tenor, who’s sung Tristan 85 times, about the demands of the role, which require both quietly resting in “screen-saver mode,” and building stamina by pumping iron at the gym.

FALA: When you say a tenor needs to be experienced to take on Wagner, what is the nature of that experience?

JT: It’s a physical experience to actually get through one of these pieces that’s five hours long. I often use the glib analogy that if you ask yourself “How will I be in five hours’ time?” can you truly answer the question. You have to build towards that in a physical, mental and vocal way. You need to know yourself before you begin taking on these massive parts. Otherwise they overwhelm you and spit you out pretty quickly. And we’ve seen some casualties along those lines.

FALA: This particular opera is notorious for actually driving tenors insane.

JT: Indeed, the very first Tristan did not live so long after his first performance.

FALA: How do you train your physical stamina for the role of Tristan?

JT: I work out three times per week on cross training machines, and do weight lifting.

FALA: Was there ever a time in the fourth hour when you began to worry about continuing? And what was the concern, your voice? Physical exhaustion? Feeling unable to sustain the intensity of performance?

JT: The answer is candidly yes, and it’s a combination of those things. But this is what I’m talking about when I mention experience, and knowledge of oneself, so that when that happens you have a contingency plan. And this is what Wagner taught me: They’re such long songs, you just have to back away a little and give yourself space. As a less experienced person, you want to just pile on the energy and hope for the best.

FALA: What do you mean by giving yourself space?

JT: Taking the whole center of the being down and seeing if you have regenerative powers right there and then. I’ve actually done it. Instead of letting my body temperature rise and getting terribly worried, thinking, “My God, how am I going to get through this?” I just really centered my mind.

FALA: A kind of zen focus, or “being in the zone,” as athletes say.

JT: Yes, and becoming incredibly centered on where each tone is going.

FALA: How do you prepare the day of a performance?

JT: First off, I rarely go out the night before. Then, within a few seconds of waking up in the morning, I know if I’m vocally fit. I’m either fit or ill, there’s nothing in between for me. Then I go into what my wife calls screen-saver mode, saving my energy.

FALA: Many have called this the most erotic work in Western art, and the greatest testament to romantic love alongside “Romeo and Juliet,” and its hypnotic effect on people like Nietzsche, Mahler and Thomas Mann is well chronicled. What’s it like to actually act out that eroticism, singing those climactic arias with such feverish music behind you?

JT: This is our fundamental task, to bring those emotions to the audience as vividly as we can. And for me, that’s an abject process of de-Treleavenization. And the more I do that, the closer I get to the piece and to giving what you paid for. I can get swept away when I’m watching “Tristan,” but when I’m performing I’m too concerned with the practical elements of it.

FALA: You’ll be playing Siegfried in LA Opera’s “Ring” cycle next year. How do these two roles stack up?

JT: They’re equally hard, though for different reasons. Siegfried is higher: You need to have started as a tenor to sing it. And of course he’s completely different in terms of emotion: He’s a young man with no history, who never looks back. It’s a deeply exciting figure to play when you reach middle age, almost a Peter Pan thing.

FALA: Tell us about this particular production.