Last night's episode revealed that Derek Branning (Jamie Foreman) has been having a secret affair with Alfie's wife Kat (Jessie Wallace) over the past six months.
Richie stated that he and his fellow actors did not know of the culprit's identity for a long time.
"We always knew there was going to be a pay-off and we knew it was going to be one of five," he told the BBC's EastEnders blog.
"When we ruled out Chucky Venn (Ray) and Steve John Shepherd (Michael), it was only one of three, and for a long time we genuinely didn't know. I'm not even sure if the culprit knew.
"So it was interesting to play for me as an actor. I loved it because Alfie's not supposed to know. But of course for Jessie, she was torn between whether she wanted to know or not, because if she had known, she was frightened she might give it away by just a look."
On who he would like to have been the culprit, Richie said: "I'd love it to have been Patrick. He's still got a few years to go yet!
"I could have justified it if it was Jack, because of the baby swap and all of that. Alfie just can't understand that it's Derek and that's what kills him. I mean, even Michael Moon because of what's happened in the past. But Derek… that just makes him sick.
"We kept saying, 'Wouldn't it have been great if it was Tanya!'"
Speaking about why Alfie demanded to know the name of Kat's lover, he said: "It's one thing Kat going with a stranger - if it's a one night stand with some bloke in Catford, he can forgive her for it.
"But now he's starting to put the jigsaw together – this guy paid him to leave the pub, he got Kat's ring from the pawn shop and gave it back, he sent her flowers, he rented a flat round the corner from The Vic!
"So there's a finality to it when Alfie says 'I don't care - you're dirty, you're disgusting, it's over. Now it's a separate issue – I need to know who it is'."
When asked why Kat decided to have an affair with Derek, Richie added: "We find out why later on. I think it's because Derek is the total polar opposite of Alfie. She's attracted to danger.
"Back in the day it was Andy Hunter. Alfie is anything but. You know exactly where Alfie is going to be the next day. You know Alfie's going to love you in a year's time. You know Alfie will do anything for you.
"Maybe it's the reverse of that situation – keep 'em mean to keep 'em keen. Alfie doesn't do that. He doesn't buy into that. He believes in honesty. He believes in treating every woman like a princess.
"And that's what he did with Kat. She's used to danger, she's used to excitement. What she got from Alfie was the fairytale, what she wanted was the nightmare."
Tonight's episode (Friday, December 21) will see the aftermath between Derek and Alfie in The Vic, airing at 8pm on BBC One.