The Blue popstar, who is on the judging panel with Anastacia, Chucky Klapow and Tamzin Outhwaite, said that it is important to tell the truth to the acts on the show.
Speaking on BBC Breakfast, James said: "I am just going to be the honest guy. I will just say it like it is and take it from there.
"It's quite fun because the audience want to get involved and boo. But you can't be nice to everybody, you can't sugarcoat everything. You just have to tell people what you think."
Singer Station and Three Spires have won the first two heats of the Sunday night show and will take part in the live final for the chance of winning an album and distribution deal.
Don't Stop Believing continues next Sunday at 8pm on Five.