The 'Taken' star was left heartbroken when he switched off Natasha Richardson's life support in 2009 following a tragic ski accident and he admits he still expects to "hear her" walk through the door at times.
He said: "Her death was never real. It still kind of isn't. There's periods now when I hear the door opening, especially the first couple of years ... anytime I hear that door opening, I still think I'm going to hear her."
The 62-year-old actor also opened up about the devastating moment he realized it was time to end her life after doctors told him his wife was brain dead.
He explained to Loaded magazine: "I went in to her and I just told her I loved her. I said, 'Sweetie, you're not coming back from this.'
"She and I had made a pact. If any of us got into a vegetative state we'd pull the plug.
"So when I saw her and saw all these tubes and stuff that was my immediate thought, 'OK, these tubes have to go. She's gone'."
However, Liam - who has sons Micheál, 18, and Daniel, 17, with Natasha admits throwing himself back into work hasn't helped ease the pain.
He said: "It hits you. It's like a wave. You just get this profound feeling of instability.
"The earth isn't stable any more and then it passes and it becomes more infrequent, but I still get it sometimes."