The actor made his debut as Luc in November and fans have already seen that the hospital's new arrival hates alcoholics for reasons unknown.
Speaking on This Morning today, Millson explained: "What's brilliant is that when I had my sit-down chat with the producers and we discussed what we should do, everything came up with question marks.
"They said, 'I know - why don't we make him a big fat question mark?' I said, 'I like that - why don't we go for Compo's matchbox?'
"Do you remember in Last of the Summer Wine, Compo used to occasionally, every few episodes, sidle up to someone and get his matchbox out, show them something and they'd scream and run away? You'd never, ever find out what was in the matchbox.
"So he's just got lots and lots of Compo's matchboxes. So I said, 'Only if we never find out some of these things'. So the alcohol thing, I wonder if we'll ever know. I certainly don't know and I don't need to know to do it - he just hates alcoholics."
Millson also discussed the spark between Luc and Eddi McKee (Sarah-Jane Potts), which will develop further in upcoming episodes.
"They're not an obvious couple," he said. "They're both dreadful socially - but in really different ways, so they are actually the right people for each other. I can't tell you what happens, because I'm not quite sure!"
Holby City continues tonight at 8pm on BBC One.
> Read our spoiler for tonight's episode of Holby City
> Read more Holby City spoilers and news