The genius psychologist will be locked up in a mental institution in tonight's instalment, titled 'Funhouse'.
"It was something that came up very early and I always liked the idea of him being in a mental institution," Roth told the Los Angeles Times. "I kind of thought that [with] his behaviour, it would be difficult to see a change in him in that place. He would look at home, up to a point, in that place."
The actor confirmed that Caprica star Paula Malcomson will appear as Lightman's late mother in a hallucination sequence.
"[Lightman] is drugged and he is hallucinating," he explained. "At one point, he sees his mum and at another point, he sees his dad, and they have conversations."
Roth also admitted that he is keen for Malcomson to reprise her role in the future.
"She's a very good actor [and] I really enjoyed working with her," he said. "It would be a great character to bring back, if he has regular conversations with his mum!"
Lie To Me returns to Fox with a double-bill tonight at 8/7c.