Max was up to no good in Manchester last year after taking time away from Albert Square when Joyner's character Tanya rejected him.
Tomorrow's (June 22) episode of the Walford soap sees Max begin to open up to Tanya about the mysterious trip, confessing that he is not proud of his actions as somebody got hurt. However, the full story remains under wraps for the time being.
Joyner appeared on Daybreak this morning (June 21) and confirmed that Max was up to "mischief" during his absence.
She commented: "We don't know what mischief. But we do know that while he was in Manchester, he got up to something.
"Do you know what, I honestly genuinely don't even know [what it is] yet! I don't know if they know yet! But there will be something. And yes, it will come out in a couple of months."
Pictured: Jo Joyner on Daybreak today.
Joyner went on to praise the other dramas in store in tomorrow night's episode, which centres around Michael and Janine's wedding.
She said: "The wedding looks fantastic. I mean, just having that marquee in the middle of the Square is unusual, isn't it? It looked brilliant - it was great. It was up for a few days and they were intensive days of filming because there was a lot of cast involved.
"It's great storylines. I mean, it's great to see Gillian [Wright] doing so much, because I just think she's fantastic. And Charlie [Brooks] is fantastic obviously - whoever she's in a relationship with, I always want to watch it. So it is all going to come to a head over the next couple of days, and it's great."
Joyner added that she is expecting busy times at work before she takes a break from EastEnders in 2013.
"I'm taking a bit of time off, not until next year though," she said. "And between now and next year, I've just been told actually - literally just been told - how busy we're going to be.
"There's going to be a lot going on for the whole of the Square, but the Brannings particularly. I can't say much more than that, isn't it annoying? There's a few things that I want to blurt out, but I can't!"
EastEnders airs an hour-long episode tomorrow from 7.30pm on BBC One.