Dan Hamilton actor Astill, who is leaving the series later this year, said that his alter ego will battle his sexuality when old friend Simon arrives at the hospital as a nurse.
"There was certainly something between them, they were good friends and that turned into something a lot stronger and deeper, but even back then Dan wasn't ready to come out and admit to whatever homosexual feelings he might have had," Astill told TV Times.
"It's a mixture of emotions because he hasn't really admitted to himself that he's gay and when Simon arrives it suddenly brings back a lot of feelings."
Meanwhile, Nicholls revealed that his character Simon puts his negative memories to one side when Paul asks him out on a date.
"Simon fell in love with Dan, whether anything physical happened we don't know, but he did fall for him and he has always been the one that got away ever since," Nicholls said.
"Simon quickly sees that Dan has not changed one iota and he's still completely denying that side of him. That annoys him because Dan is ignoring who he is and it puts him off him, but then one night he asks him out for a drink, and Simon says yes and then things start to happen."
Holby City continues next Tuesday at 8pm on BBC One.