Lewis bonded with the Love Actually star on the set of the wizarding tales, and headed to his trailer to bid him a fond farewell when filming ended in 2010.
Rickman encouraged the 23 year old to tackle a theatre role, and Lewis now stars in West End play Our Boys, but he admits the different style of acting came as a shock.
Lewis tells Britain's Daily Mirror, "I went to his trailer on his last day and said, 'I just want to say it's been an absolute privilege these last 10 years'. We chatted for a bit and he said I suggest you do a bit of theatre.
"But at first the acting style was so different. I was shouting in everyone's face rather than 'projecting'. Vocal warm-ups - what's that? Eventually, after four months of embarrassing the rest of the company, I figured it out."