On Monday (March 19), over 10m viewers tuned in to see Frank's mother Anne (Gwen Taylor) exposed as his killer - with dangerous consequences for Sally Webster (Sally Dynevor) and Carla Connor (Alison King).
Although Lancel had known the identity of the culprit for some time, he watched on the night to see the drama play out on screen.
Lancel told PA: "I watched the episode doing the ironing. I'm very proud of it. I thought Gwen was amazing.
"As a Corrie fan, someone who has watched it my whole life - to see these iconic characters go to this dark place that Frank got them to was a huge buzz."
Discussing the secrecy surrounding the whodunit plot, he continued: "We lie - I lied to everyone. I spoke to Gwen this morning and she's hugely relieved that it's out there because we've been lying to everybody for so long.
"I knew what was going to happen with [Frank] - what he did, who he did it to, when he was going to go and how he was going to die. For me, it was just a matter of containing it, but people really got hold of it."
Lancel finished filming the role of Frank in early February.
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> Coronation Street Gwen Taylor: 'Anne's behaviour was terrible'
> Corrie's Gwen Taylor: 'Andrew Lancel told me I was the killer'
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Watch a preview of Thursday night's (March 22) Coronation Street episode in the video below: