The group confirmed earlier this week they are planning to use vocals recorded by Stephen before his sudden and unexpected death last month on their new album, but frontman Ronan Keating said their grief may prevent them from taking the new material on tour.
He said: "We've got his vocals for several tracks, thank God. But as for going on the road, we may tour, we may not. I keep remembering things and I get emotional."
The singer also revealed he thinks the band's next single - which will feature Stephen's voice and was written by British pop star Mika - will have a huge impact.
Speaking in Melbourne, Australia, he added: "It's going to be the biggest song of next year, I promise.
"Stephen actually recorded his vocals on it, which is very exciting. He did backing vocals, he sounds like an angel, which is ironic."
The group have also spoken out about their final night with Stephen, when they stayed in the church in Dublin with his coffin on the night before his funeral.
Ronan, 32, said: "All of us are so happy that we did it. It was our last real night together as a five-piece, and it felt like that - it felt like we were quiet.
"We lay on the floor to the left of the coffin in our sleeping bags and we cried and we laughed and we could hear him telling us jokes and answering back, what he would say back.
"We didn't get much sleep - a little bit but that's not what it was about - it was just about spending time with Stephen and we knew Stephen well. He didn't like the dark, he didn't like being on his own."
Stephen died aged 33 from an acute pulmonary oedema, a build-up of fluid in the lungs, at his holiday home in Majorca on October 10.