Speaking to Entertainment Tonight, the 65-year-old actor said: "I was blissfully unaware ... I was so self-involved at that point, they could've been doing it in front of me and I wouldn't have noticed. I was single and had my own agenda ... I think it might have been a distraction if I'd known what was going on."
Hamill - who reunited with Fisher and Ford for 2015 Star Wars film The Force Awakens - admits on-set romances are quite "commonplace" but often end in awkwardness for those involved and the rest of the cast.
And the sci-fi star knows from first-hand experience because it happened to him when he had a short-lived relationship with his General Hospital co-star Anne Wyndham in the early 1970s.
Mark - who has been married to his wife Marilou since 1978 - shared: "If the relationship doesn't last and you're on a TV series and you got four more seasons to go, it can be really awkward. It happened to me. I was with a girl who was playing my sister on a soap opera, General Hospital."
In her book, Fisher, 60, spills that Hamill almost caught her and Ford, 74, in the act when he turned up unannounced at Harrison's apartment when she was there for sex.
She writes: "Once, on one of the rare occasions when we did have a sleepover at Harrison's apartment, Mark ... dropped by unannounced. Clearly I hadn't just dropped by for brunch."
- Bang! Showbiz
