LONDON — There’s no plaque to honor the encounter, and neither of its central participants can pinpoint the exact date it occurred, but somewhere on a stretch of 14th Street in Manhattan’s East Village is the spot where, in the late 1960s, two rookie actors named Robert De Niro and Al Pacino first crossed paths.

They were up-and-comers enjoying early tastes of steady work and visibility, and they knew each other by name and reputation. They compared résumés, sized each other up — Pacino still remembers De Niro as having “an unusual look and a certain energy” — and each walked away wondering what the future held for himself and the man he had just met.

A half-century later, they ambled into a suite at a luxury hotel on the River Thames to talk about their new film, “The Irishman,” with so many of those uncertainties put to rest long ago. Whatever can be achieved as an actor, De Niro and Pacino have pretty much done it, surpassing even the outsize aspirations they had as young men. They have provided cinema with some of its most transfixing and explosive protagonists, in landmark films like — let’s just get these out of the way — “Taxi Driver,” “Scarface,” “Raging Bull” and the “Godfather” series.

In doing so, their trajectories have become unexpectedly intertwined. They are not only peers and occasional collaborators but genuine friends who occasionally find time to check in, contemplate possible projects and push each other’s buttons.